In the last ten years, the dropout rate among Black high school studen

In the last ten years, the dropout rate among Black high school students fell substantially over the past decade, while the number of Blacks who attend college is more than twice what it was.

(A) fell substantially over the past decade, while the number of Blacks who attend college is more than twice what it was
(B) fell substantially, while the number of Blacks attending college is more than double what it was at that time
(C) has fallen substantially, while the number of Blacks attending college has more than doubled
(D) has fallen substantially over the past decade, while the number of Blacks attending college is more than twice what it was at that time
(E) has fallen substantially over the past decade, while the number of Blacks who are attending college are more than double what they were

Remove A, D and E for redundancy.

It looks as though choice B implies that the rate fell on a particular time in the last ten years. This is logically wrong. The drop in the rate is over a period and hence could be gradual or even erratic, but finally reaching the said the point at the end of the given period.

C says that much by using the present perfect 'has fallen'. C is also more forthright in expressing the increase as 'has more than doubled".

C is the correct choice.