问题
单项选择题 A2型题
单卵双胎时，受精卵分裂极少发生在（）。
A.桑椹期
B.晚期囊胚
C.羊膜囊形成后
D.原始胚盘形成后
E.以上都不是
答案
参考答案：C
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单卵双胎时，受精卵分裂极少发生在（）。
A.桑椹期
B.晚期囊胚
C.羊膜囊形成后
D.原始胚盘形成后
E.以上都不是
参考答案：C
Directions: There are 10 blanks in the following passage. For each numbered blank, there are 4 choices marked A, B, C and D.
| The key to the industrialization of space is the U.S. space shuttle. (51) it, astronauts will acquire a workhouse vehicle (52) of flying into space and returning many times. (53) by reusable rockets that can lift a load of 65 000 pounds, the shuttle will carry devices for scientific inquiry, as well as a variety of military hardware. (54) more significantly, it will deliver materials and machines into space for industrial purposes (55) two decades ago when "sputnik" (artificial satellite) was added to the vocabulary. In short, the (56) importance of the shuttle lies in its promise as an economic tool. What makes the space shuttle (57) is that it takes off like a rocket but lands like an air plane. (58) when it has accomplished its mission, it can be ready for (59) trip in about two weeks. The space shuttle, the world’s first true spaceship, is a magnificent step (60) making the impossible possible for the benefit and survival of man. |
A．Then
B．Or
C．But
D．So