Questions 22 to 25 are based on the following monologue. What conclusion can be drawn from the scientists’ observation

A．Insects have developed some sort of resistance to man - made poisons.
B．Insects have been eating plants for about 250 million years.
C．Farmers sometimes use a combination of two or three insecticides at once.
D．New insects mate with insects which survived from a certain kind of insecticide.