染色体是由____和____共同组成的．人的性别主要由性染色体决定，男性的性染色体是____，女性的性染色体是____．

某人从一鱼池中捕得120条鱼，做了记号之后，再放回池中，经过适当的时间后，再从池中捕得100条鱼，结果发现有记号的鱼为10条（假定鱼池中不死鱼，也不增加），则鱼池中大约有鱼（　　）

三年级同学参加戏曲小组的人数是参加体育小组人数的 $\frac{3}{5}$ 参加体育小组的一共有20人。

    Good habits are important to us. We should build good living habits. Now let's have a look at the two brothers. Li Qiang and Li Gang are brothers. But their living habits are different. Now let's go to Li Qiang's room. Li Qiang cleans his bedroom every day and puts his things away. Look! There is a computer on the desk. There are two balls under the bed. A guitar and a kite are on the wall. Some books are on the shelf. So he must be a careful boy. Now let's go to Li Gang's room. The room is not clean. Can we see his clothes on the desk? Oh, yes. We can see a football and a knife on the desk, too. Li Gang never does any cleaning on weekends. So his room is in a mess.

    What do you think of Li Qiang and Li Gang? Li Qiang is a good boy. Li Gang must learn from his brother