听力原文：W: Dr. Cottam. You're studying the effects of air pollution on trees？
M" That's right.
W: Could you tell us something about it？
M: Trees in industrial areas grow much more slowly than they should. Worse still, trees hundreds of miles away also seem to be damaged by these same industries.
W: You mean "acid rain", I suppose.
M: Yes.
W: What effects does acid rain have on the environment'？
M: For a start, lakes polluted by it in this way can no longer support any fish. Gradually the acid rain washes elements out of the soil essential for healthy plant growth.
W: But I think that there's no general agreement on the causes and effects of acid rain.
M: That's right. The distances involved are so great that it's impossible to prove what's actually going on. And no one will willingly accept even partial responsibility for something that happens miles away.
W: But trees are dying. No one can deny that.
M: Exactly.
W: So how does your investigation go？
M: We've set up special glasshouses to keep a careful observation on the growth of the plants. We pump in controlled amounts of air pollutants, including ozone, which isn't always associated with acid rain.
W: Which industries emit ozone？
M: Mainly oil refining industry. We also occasionally cool the atmosphere in the greenhouses, which causes frost.
W: Have you had any results from these experiments yet？
M: It's still too early to draw any definite conclusions. But we've noticed that plants at low temperatures are affected more severely than those at normal temperatures.
W: So cold weather makes the effects of pollution worse？
M: It certainly looks that way. But we still hope to be able to persuade industries to take measures to reduce emissions of gases into the atmosphere.
(20)

A. Fishes die in lakes.
B. Elements needed for healthy plant growth are washed out of soft.
C. Trees grow much more slowly than they should.
D. It contributes to climate change and causes frost.