A、It limited their supply of food. B、It made their eggshells too fragile. C、It destroyed many of their nests. D、It killed many b

America’ s national symbol, the bald eagle, almost went extinct twenty years ago, but it has made a comeback. In fact, the U. S. Fish and Wild Life Service is 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 1960 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 1972, a law was passed to ban DDT, which saved the bald eagle from total wipeout. And since then wild life biologists had reintroduced bald eagles from Canada to America. The result was that last year U. S. bird watchers counted eleven thousand 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.
Q:
16. What was the main harmful effect of the pest killer DDT on bald eagles?
17. What measure did the wild life biologists take to increase the number of bald eagles?