Insects are also interesting. First, they are very 76 ____animals. Three hundred and twenty million years ago, when no men or other 77 ____lived in the world, there were insects. Today, in every square mile of land there are millions of them flying and crawling 78 ____. Second, insects are so 79 ____to their habitat, to their food and to the weather that now there are about a million different species in the world.
However, 80 ____do people use insecticides (杀虫剂)? In many cases we do not see the insects and we do not think much about them. The 81 ____is that insects eat so much of man’s food. We must use insecticides to 82 ____them.
An insecticide is simply a special chemical 83 ____with some poisonous elements in it. Farmers 84 ____their crops very often and the insects die quickly. Of course some poisonous chemicals may stay on the crops or in the soil, 85 ____is also dangerous.85____
A、 this
B、 which
C、 so
D、 it
【正确答案】：B
