用方框内所给词的适当形式完成下面短文（有一项多余）

cause   problem   crowd   offer   a population of    five

Dear Dick,

    How time flies! I have been back to school for two weeks. These days, I have known a lot about the population of China. China has about

1.37 billion. It reaches about one of the world's population.

About 50 percent of the people live in the countryside. It many difficulties. In less developed areas, some parents prefer boys to girls. Not all the families can their children a good education. Some children can't go to school. China tried to control the population. The large population is still serious in China though we have had a more comfortable life. How about the population of your country? Can you tell me about it?

Yours,

Maria