已知马蛔虫体细胞内有2对染色体，如图示能正确表达马蛔虫生殖细胞中染色体组成的是（　　）

完形填空

Most American families are smaller than those in other countries. They have one or two （1）________ and one or two children each.

Children in the US will（2）________ their parents' house when they grow up. They usually live（3）________ their parents because of work. They often（4）________ to their parents or telephone them. And they often go to visit their parents when they are（5）________ vacation.

Parents usually let their children choose their（6）________ jobs. Americans think it's （7）________ for young people to decide their future by themselves.

Parents also （8）________ their children to do some housework at home. And in many families, children are paid for （9）________ housework. Parents think it can help them learn（10）________ to make money for their own use.