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 Correct answer - no apparent errors.

(B) that of their estimated population "that of their" illogically refers to a possession of "their" ie; something possessed by the black rhino population. Eliminate.

(C) more than the population's estimated amount Incorrect usage of "amount" for a countable. Eliminate.

(D) more black rhinos than their population estimate "Population" is being incorrectly compared to "estimate". Eliminate.

(E) more than that of their population's estimated amount Same error as in (C). Eliminate.