在正常情况下，人体精子内的染色体数目与性染色体的组成应该是（ ）
A．22条+XY
B．22条+Y
C．22条+X
D．B或C
在亲代的生殖细胞形成过程中，经过减数分裂，染色体彼此分离，男性产生两种类型的精子--含22+X染色体的精子和含22+Y染色体的精子．女性则只产一种含22+X染色体的卵细胞．受精时，如果是含22+X的精子与卵子结合，就产生具有44+XX的受精卵并发育成女性；如果是含22+Y的精子与卵子结合，就产生具有44+XY的受精卵并发育成为男性．因此，在正常情况下，人体精子内的染色体数目是22条+Y或22条+X．遗传图解如图：
故选D
| Everyone wants to stay healthy, and they usually have different ways. Some people do exercise every day, and others keep a balanced diet. As for me, I choose(选择)the latter. I'm daydaytest Placeholder! The most important reason for keeping a balanced diet is that it can give you all the nutrition (营养)your body needs. You should change your diet every other day and it should have rice, meat, vegetables, fruit, milk and so on. That's because these foods will help you stay in good health. The second reason for keeping a balanced diet is that it won't take you much time to do it. Finally, you can enjoy different kinds of delicious food and keep healthy at the same time. So I think the best way to stay healthy is to keep a balanced diet, which will also lead to a happy life. | |||||||
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| 1. To stay healthy, the writer chooses to ________. | |||||||
| A. do exercise every day B. enjoy delicious food C. keep a balanced diet | |||||||
| 2. Different kinds of _______ will help you stay in good health. | |||||||
| A. exercises B. food C. lifestyles | |||||||
| 3. It will take you ______ time to keep a balanced diet. | |||||||
| A. a little B. lots of C. little | |||||||
| 4. The best title of the passage should be "______". | |||||||
| A. Having good living habits B. The best way to stay healthy C. How to keep healthy |