A、DDT. B、The disturbing of nests. C、Hunting. D、The destruction of their natural homes. D短文末尾指出，自然家园受到破坏可能会成为下一个“DDT”，使秃鹰的数量锐减。因此

America’s national symbol, the bald eagle, almost went extinct 20 years ago. But it has made a comeback. In fact, the US Fish and Wildlife Service is considering the possibility of taking it off the en-dangered-species list.
    Once, more than 50,000 pairs of bald eagles nested across the country. But by 1960, that number had fallen below 400. The chief killer was the widely used DDT. Fish soaked up DDT, died, and were washed up on shores, where bald eagles ate them up. DDT caused eagle eggshells to weaken and crack. The shells became so thin that they cracked before the babies emerged from the eggs. The eagle had no defense against DDT.
    Fortunately, in 1972, a law was passed to ban DDT, which saved the bald eagle from dying out. And since then, wildlife biologists have reintroduced bald eagles from Canada to America. The result was that last year U.S. birdwatchers counted 11,610 bald eagles in the country. If it were dropped from the endangered 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.
16. What do we learn about the bald eagle?
17. How did DDT affect the bald eagles?
18. What has been done to prevent bald eagles from dying out?
19. According to the passage, what will cause the number of bald eagles to decline?