本初子午线是指（　　）

    There are forty-eight students in our class. There are twenty-three boys and twenty-five girls. Some students live near the school, and some others live very far from the school. About half of the students come to school by bike. They often get to school at a quarter to seven. About ten students often come to school by bus. They often go to school very early, too. Another ten students come to school on foot. Their homes are near the school, but they are often late for school, because they get up very late.

非洲主要的地形类型是（  ）