Science has told us so much about the moon that it is quite easy to form an idea of what it would be like to go there. It is certainly not a friendly place. As there is no air or water, there can be no life of any kind. Mile after mile there are only plains(平原) of dust with mountains around them. Above, the sun and stars shine in the black sky. If you step out of the mountains shadows, it will mean moving from terrible cold into great heat. The moon is also a very silent(安静的) world, for sound waves (声波)can only travel through air. But from the moon, you can see a friendly sight(景色). Our earth is shining more brightly than the stars. It looks like a large ball, coloured blue and green and brown.

个婴儿不能消化乳类，经检查发现他的乳糖酶分子有一个氨基酸改换而导致乳糖酶失活，发生这种现象的根本原因是(　　)