题干

下图为人体某早期胚胎细胞所经历的生长发育阶段示意图,图中①~⑥为各个时期细胞,a~c表示细胞所进行的生理过程。据图分析,下列叙述正确的是(   )

A:与①细胞相比,②细胞的相对表面积减小,物质运输的效率减弱

B:③④细胞均来源于早期胚胎细胞减数分裂,遗传物质不同,基因表达产物相同

C:⑤⑥细胞发生了细胞分化,这是由于⑤⑥细胞中的遗传物质的改变

D:人在胚胎发育后期尾的消失是由于尾部细胞衰老坏死而实现的

上一题 下一题 0.0难度 选择题 更新时间:2020-03-06 05:48:13

答案(点此获取答案解析)

A

同类题2

阅读理解

    One year, a college in the United States announced that it would offer a lot of money for the pure white marigold(金盏花). The high reward attracted so many people, but in the nature, besides golden, the marigold is brown, and it is not easy to get the white one. So after they were excited for a time, many people had forgotten the announcement.

    One normal day after 20 years, the college received a letter and 100 seeds of pure white marigold. It was an old woman of over 70 years old. Some experts in the college doubted it, but in order not to let her down, those seeds finally took root (根)in the earth. The miracle appeared after one year; the field was covered by pure white marigold.

    Therefore, the old woman who was always unknown to the public became a new focus.

    The old woman was a flower-lover. When she read the announcement 20 years ago, she got very excited like others. But her eight children were totally against her decision. After all, a woman who never knew the seed genetics(遗传学) couldn't complete what the experts could never do, so her thought was only a day dream.

    Still, the old woman didn't change her mind and went on working. She planted some of the most common seeds and took good care of them. A year later, when the marigold came out, she chose one faintest(最暗淡的) from those golden and brown flowers and got the best seed. The next year, she again grew them and chose...Day after day, year after year, through many seasons, the old woman's husband died, her children flew far, a lot of things happened in her life, but only the wish to grow the pure white marigold took root in her heart.

    Finally, after 20 years on the day we all know, in the garden she saw a marigold, which was nearly white, but as white as silver or snow.

    A problem even experts couldn't deal with was solved by an old woman who didn't understand genetics. Was it a miracle  (奇迹)?

To take root in the heart, even the most common seed, can grow into a miracle!