A、By cutting a path all the way across the leave. B、By eating only the stem of the plant. C、By changing the plant’ s chemistry.

This morning I want to tell you about a recent scientific discovery dealing with the relation between plants and animals. This is about a desert shrub whose leaves can shoot up a stream of poisonous resin a distance of six feet. You think it would be safe from all attacks by insects. But a recent study has found one insect, a beetle, that can chew its way pass the plant’s defense system by cutting the mainly vein that delivers the poison to the leaves. This vein cutting is just one method the beetles used to prepare a safe meal. Another is by cutting a path all the way across the leave to hold the flow of chemicals. Then they simply eat between the veins of poison. In the past, scientists who studied insect adaptation to plant defenses have focused on chemical responses. That is, how the insects can neutral or alter the poisonous substances plants produce. What’s unique about this chewing strategy is that the beetle is actually exhibiting a behavior response to the plant’s defenses rather than the more common chemical response. It is only after a beetle’s survived several encounters with the plant’s resin that it learns how to avoid the poison: by chewing through the resin transporting veins on the next leaf it eats. And thus gives itself a safe meal, however, it can take a beetle an hour and a half of careful vein cutting to prepare a small leaf that takes it only a few minutes to eat ,so, though the method is effective, it’s not very efficient. Now listen to the following recording and answer questions 19 to 21.
19. What is the talk mainly about?
20. What is unusual about the desert plant?
21. Except vein cutting How can the beetles avoid being poisoned by the plant?