Questions 17 to 20 are based on the following passage. At the end of the prorate, you will be given 15 seconds to answer the questions. Now listen to the passage. 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.