Today Newton is a very clean place. Many years ago, however, there were millions of rats in it. They attacked the cats and dogs. Sometimes a great number of them knocked down a man or woman walking home at night. The rats were very large in size and they harmed many people.
The Government ordered everybody to kill rats. Most people were lazy, so they didn't kill many. The Government promised to pay some money for each dead rat. That made the people very happy. They killed thousands of rats everyday. A Government officer put all the dead rats in a big pile. Sometimes a man brought hundreds in one day.
After two weeks there were not many rats in the city, but people still brought many rats to the Government office. The Government officer thought that people were stealing dad rats from the pile. He ordered his men to dig a deep hole and put the rats in it. Soon there were no more rats, and the Government didn't pay any more money.
3. The people killed rats ______.

A.to get money from the Government

B.to help the Government make the city clean

C.to make the Government officer happy

D.to protect（保护）their cats and dogs