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. Why did the resistance of insects to man-made insecticides develop so rapidly

A．Because farmers use less insecticides now.
B．Because farmers have been using insecticides for many years.
C．Because insects arc using the mechanisms against poisons produced by plants to deal with insecticides.
D．Because insects have been eating plants for many million years.