Africa's black rhino population in the mid-1970's numbered about 20,000, ten times the estimated population of 2,000 in 1997.
(A) the estimated population
(B) that of their estimated population
(C) more than the population's estimated amount
(D) more black rhinos than their population estimate
(E) more than that of their population's estimated amount 答案是a，但问题是，ten times 之后的用法我不熟悉（必须要用than吗），而且，2000是一个数字，应该用amount修饰才对吧。。。。 应该还是times用法的问题吧。。。。。