正确答案：CE
解析：
This is a classic time shift: the blank is the opposite of 'seriously diminished...'. Thus we want a word that means a great number of.

(A) sparse is the opposite of dense
(B) eradicated means wiped out
(C) innumerable means a great number
(D) immutable means cannot be changed
(E) legion means great in number
(F) thoroughgoing means absolute, which is not the same as having a great number of