下列过程中，主要依靠化学变化实现的是
     

从小学升上中学，意味着我们的人生又迈上了一个新台阶，我们的生活发生了很大变化，面对新的开始，我们应该（ ）

阅读理解。

    Amy and Sarah are good friends. They are from America. They like winter in America because there are many activities in winter. They can make a snowman,go fishing and go skiing with many friends. There are two festivals (节目) in winter. They're Christmas Day and Thanksgiving. Amy and Sarah are very happy. But they're in Beijing now. They like winter in Beijing, too. Because It's beautiful. It's windy and cold. They're going to visit the Great Wall, so they're very happy.

当一个婴儿体细胞中第13对染色体多了一条时，就会导致先天性唇裂．在下列生殖细胞中，哪两种生殖细胞的结合会产生先天性唇裂的男性患儿（A表示常染色体）？（　　）

①23A﹢X    ②22A﹢X    ③21A﹢Y     ④22A﹢Y．