bbin注册网址

    <dir id='ul8s6'><del id='ul8s6'><del id='ul8s6'></del><pre id='ul8s6'><pre id='ul8s6'><option id='ul8s6'><address id='ul8s6'></address><bdo id='ul8s6'><tr id='ul8s6'><acronym id='ul8s6'><pre id='ul8s6'></pre></acronym><div id='ul8s6'></div></tr></bdo></option></pre><small id='ul8s6'><address id='ul8s6'><u id='ul8s6'><legend id='ul8s6'><option id='ul8s6'><abbr id='ul8s6'></abbr><li id='ul8s6'><pre id='ul8s6'></pre></li></option></legend><select id='ul8s6'></select></u></address></small></pre></del><sup id='ul8s6'></sup><blockquote id='ul8s6'><dt id='ul8s6'></dt></blockquote><blockquote id='ul8s6'></blockquote></dir><tt id='ul8s6'></tt><u id='ul8s6'><tt id='ul8s6'><form id='ul8s6'></form></tt><td id='ul8s6'><dt id='ul8s6'></dt></td></u>
  1. <code id='ul8s6'><i id='ul8s6'><q id='ul8s6'><legend id='ul8s6'><pre id='ul8s6'><style id='ul8s6'><acronym id='ul8s6'><i id='ul8s6'><form id='ul8s6'><option id='ul8s6'><center id='ul8s6'></center></option></form></i></acronym></style><tt id='ul8s6'></tt></pre></legend></q></i></code><center id='ul8s6'></center>

      <dd id='ul8s6'></dd>

        <style id='ul8s6'></style><sub id='ul8s6'><dfn id='ul8s6'><abbr id='ul8s6'><big id='ul8s6'><bdo id='ul8s6'></bdo></big></abbr></dfn></sub>
        <dir id='ul8s6'></dir>
      1. <small id='2ti8m'></small><noframes id='2ti8m'>

      2. <tfoot id='2ti8m'></tfoot>

          <legend id='2ti8m'><style id='2ti8m'><dir id='2ti8m'><q id='2ti8m'></q></dir></style></legend>
          <i id='2ti8m'><tr id='2ti8m'><dt id='2ti8m'><q id='2ti8m'><span id='2ti8m'><b id='2ti8m'><form id='2ti8m'><ins id='2ti8m'></ins><ul id='2ti8m'></ul><sub id='2ti8m'></sub></form><legend id='2ti8m'></legend><bdo id='2ti8m'><pre id='2ti8m'><center id='2ti8m'></center></pre></bdo></b><th id='2ti8m'></th></span></q></dt></tr></i><div id='2ti8m'><tfoot id='2ti8m'></tfoot><dl id='2ti8m'><fieldset id='2ti8m'></fieldset></dl></div>

              <bdo id='2ti8m'></bdo><ul id='2ti8m'></ul>

              1. <form id='qoq9u'></form>
                  <bdo id='qoq9u'><sup id='qoq9u'><div id='qoq9u'><bdo id='qoq9u'></bdo></div></sup></bdo>

                    • 无忧雅思网_雅思预测_雅思机经_雅思考试_雅思资料下载_雅思名师_2018年雅思考试时间

                      托福考试:TOEFL模拟试题

                        Questions 12-20

                        The elements other than hydrogen and helium exist In such small quantities that it is accurate to say that the universe somewhat more than 25 percent helium by weight and somewhat less than 25 percent hydrogen.

                      bbin注册网址   Astronomers have measured the abundance of helium throughout our galaxy and in other galaxies as well. Helium has been found In old stars, in relatively young ones, in interstellar gas, and in the distant objects known as quasars. Helium nuclei have also been found to be constituents of cosmic rays that fall on the earth (cosmic "rays" are not really a form of radiation; they consist of rapidly moving particles of numerous different kinds). It doesn't seem to make very much difference where the helium is found. Its relative abundance never seems to vary much. In some places, there may be slightly more of it; In others, slightly less, but the ratio of helium to hydrogen nuclei always remains about the same.

                        Helium is created in stars. In fact, nuclear reactions that convert hydrogen to helium are responsible for most of the energy that stars produce. However, the amount of helium that could have been produced in this manner can be calculated, and it turns out to be no more than a few percent. The universe has not existed long enough for this figure to he significantly greater. Consequently, if the universe is somewhat more than 25 percent helium now, then it must have been about 25 percent helium at a time near the beginning..

                        However, when the universe was less than one minute old, no helium could have existed. Calculations indicate that before this time temperatures were too high and particles of matter were moving around much too rapidly. It was only after the one-minute point that helium could exist. By this time, the univ

                        erse had cooled sufficiently that neutrons and protons could stick together. But the nuclear reactions that led to the formation of helium went on for only a relatively short time. By the time the universe was a few minutes old, helium production had effectively ceased.

                        12. what does the passage mainly explain?

                        (A)How stars produce energy

                        (B)The difference between helium and hydrogen

                      bbin注册网址   (C)When most of the helium in the universe was formed

                        (D)Why hydrogen is abundant

                        13. According to the passage, helium is

                        (A) the second-most abundant element in the universe

                      bbin注册网址   (B) difficult to detect

                      bbin注册网址   (C) the oldest element in the universe

                        (D) the most prevalent element in quasars

                      bbin注册网址   14. The word "constituents" in line 7 is closest in meaning to

                      bbin注册网址   (A) relatives

                        (B) causes

                        (C)components

                      bbin注册网址   (D) targets

                        15. Why does the author mention "cosmic rays't' in line 7?

                        (A)As part of a list of things containing helium

                      bbin注册网址   (B) As an example of an unsolved astronomical puzzle

                        (C) To explain how the universe began

                      bbin注册网址   (D) To explain the abundance of hydrogen in the universe

                        16. The word "vary" in line 10 is closest ill meaning to

                        (A) mean

                        (B) stretch

                        (C) change

                        (D) include

                        17. The creation of helium within stars

                      bbin注册网址   (A) cannot be measured

                      bbin注册网址   (B) produces energy

                        (C) produces hydrogen as a by-product

                      bbin注册网址   (D) causes helium to be much more abundant In old stars than In young star:

                      bbin注册网址   18. The word "calculated" in line 15 is closest in meaning to

                        (A) ignored

                        (B) converted

                        (C) increased

                      bbin注册网址   (D) determined

                      bbin注册网址   19. Most of the helium in the universe was formed

                        (A) in interstellar space

                      bbin注册网址   (B) in a very short time

                        (C) during the first minute of the universe's existence

                        (D) before most of the hydrogen

                        20. The word "ceased" in line 26 is closest in meaning to

                      bbin注册网址   (A)extended

                        (B)performed

                        (C)taken hold

                        (D)stopped

                        Questions 21-30  In colonial America, people generally covered their beds with decorative quilts resembling those of the lands from which the quitters had come. Wealthy and socially prominent settlers made quilts of the English type, cut from large lengths of cloth of the same color and texture rather than stitched together from smaller pieces. They mad these until the advent of the Revolutionary War in I 775, when everything English came to be frowned upon.

                        Among the whole-cloth quilts made by these wealthy settlers during the early period are.

                      打鱼棋牌游戏---首页_欢迎您 两张棋牌---HOME_Welcome 英生棋牌---首页_欢迎您 天际亚洲---首页_欢迎您 金胜棋牌---首页_欢迎您

                      打鱼棋牌游戏---首页_欢迎您 两张棋牌---HOME_Welcome 英生棋牌---首页_欢迎您 天际亚洲---首页_欢迎您 金胜棋牌---首页_欢迎您