【单选题】

{{I}}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.{{/I}} 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.