综合填空。

Dear Dick,

     How time flies! I have been back to school 1          two weeks. These days, I have 2           a lot about

the population in China.

     China has a population of 1.3 billion. It has 3          about one fifth of the world's population. About 80

percent of the people live in the 4          . It causes many difficulties. In less 5          areas, some parents

6          boys to girls. Not all the families can offer their children a good 7          . Some children can't go

to school.

     China has taken measures to 8          the population. China's economy grows 9          faster and their

living conditions have changed a lot. Most of us have lived in bright and tall buildings. We have had ring

roads and they are wide enough. We have had more comfortable life.

     However, the large population problem is still 10           in China.

     How about the population of your country? Can you tell me about it?

                                                                                                                                               Yours,

                                                                                                                                               Maria

从组成母体的若干分批中抽取一定数量的分批，然后再从每一分批中随机抽取一定数量的样本是( )。

A．系统抽样法

B．二次抽样法

C．分层抽样法

D．单纯随机抽样法