The gyrfalcon, an Arctic bird of prey, has survived a close brush with extinction; its numbers are now five times greater than when the use of DDT was sharply restricted in the early 1970's.
这道题是正确答案,但是不觉得greater than 后面when前面 应该加一些东西吗?
我觉得挺奇怪的。也许可以是its numbers are now five times greater than its numbers when ? 难道就省略掉了?