【正确答案】
C
【答案解析】[听力原文]16-20
M: The volcanic ash from the eruption of Mount Saint Helens certainly caused a lot of damage, didn't it?
W: It did, but not as much as experts had predicted. It seems to have had a beneficial effect as well.
M: Really? But were crop losses great?
W: Yes, However, since the weather following the eruption was unexpectedly good, some crop yields remained steady or even increased beyond normal expectancy levels.
M: I'm glad to hear that. But what was the beneficial effect you mentioned?
W: A mass destruction of various insect populations.
M: How could the ash kill insects without effecting people too?
W: It's highly abrasive and quickly wears away the outer wax layer protecting insect bodies. Re- moval of that layer causes the insects to lose body moisture by evaporation, the result is usual- ly lethal.
M: How about that? Sounds like an effective yet natural insecticide.