A、Using less insecticide. B、Giving the plants up. C、Spraying crops frequently. D、Trying other insecticide. A短文提到了科学家给农场主的三点建议：少用

Scientists have observed that plants themselves produce many poisonous chemicals for defense against insects. But insects have developed ways of dealing with them and, in fact, have been eating plants for about 250 million years. Now insects are using these same mechanisms to deal with man-made poisons—insecticide. This is why resistance of insects to insecticide has developed so rapidly. What should be done? Scientists studying the problem suggest that farmers use less insecticide. At the moment, farmers regularly spray crops as a precaution against problems that are caused by large numbers of pests. They should, instead, spray only where pests have actually been seen. Secondly, farmers could use a combination of two or three insecticide at once. To survive, the insects would need to become resistant to two or more poisons at the same time. The most surprising suggestion, perhaps, is that, at certain times of the year, farmers should actually try to attract new insects onto the crops they are trying to protect. The new insects will mate with those which survived from a certain kind of insecticide and will lessen the latter’s resistance to it.
    Scientists hope that these and other measures will postpone the day when farmers and scientists will have to stand by while new super bugs which may be resistant to all our poisons invade our farms and devour our crops.
19. What does the speaker say about plants?
20. What is mentioned as one of the suggestions raised by scientists?
21. What is the most surprising suggestion scientists give?