单项选择题

B

It was summer in a small town in Illinois, and it was hot. Insects were all around. They were outside! They were inside! They bit people! They crawled on food! The people didn’t know what to do. Some people wanted to kill the insects with sprays. Others said, "Insect spray will harm the birds and the butterflies. That is a bad idea." Someone said, "Birds can help. Purple martins eat insects. We need purple martins." The people made bird houses and put them along the streets. Soon purple martins built nests in the bird houses. They had baby birds and fed them insects. As the babies got bigger, they ate more and more. The people of the town were happy. They had found a good way to get rid of the insects and not harm the birds and the butterflies. Now there are bird houses in many places and hardly any more insects in the town.

The main idea of the whole story is that ______.