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听力原文： America's national symbol, the bald eagle, almost went extinct twenty years ago, but it has made a comeback. In fact, the U. S. Fishing and Wild Life Service are considering the possibility of
taking it off the Endangered Species List. Once, more than fifty hundred pairs of bald eagles nested across the country, but by 1920 that number had fallen below four hundreD.The chief killer was the widely used DDT. Fish soaked up DDT, died, and were washed up on shores where bald eagles feasted on them. DDT prevented eagle egg shells from thickening. The shells became so thin that they shattered before the babies hatcheD.Fortunately in 1.932, a law was paused to ban DDT which saved the bald eagle from total wipeout. And since then wildlife biologists have reintroduced bald eagles from Canada to AmericA.The result was that last year U. S. bird watchers counted eleven thou- sand six hundred and ten bald eagles in the country.
If it were dropped from the Endangered Species List, the bald eagle would still be a threatened species. That means the bird would continue to get the same protection. No hunting allowed, and no disturbing of nests. But bald eagles still face tough times. The destruction of their natural homes could be the next DDT causing eagle numbers to drop quickly.
What was the main harmful effect of the pest's killer DDT on bald eagles?
A.It limited their supply of fooD.
B.It destroyed many of their nests killed
C.It many baby bald eagles.
D.It made their eggshells too fragilE.