题目

马是哺乳动物，其体细胞内有64条染色体。在正常情况下，马的精子内常染色体和性染色体的数目和类型是 ①64+XY     ②62+XX     ③31+X     ④31+Y A．③              B．④            C．①②           D．③④ 答案：D 下列各句中均有一处错误，请指出并改正。 1. Three-year-olds is too young to dress themselves. 2. The boy is not enough old to be a volunteer. 3. You can’t allowed to go boating today. 4. The teachers come from England. So are the students.5. I’m not allowed to go out on school nights too. _________________________