verbal 2nd 29 it的指代问题

According to some economists, the julydecrease in unemployment so that it was the lowest in two years suggests that the gradual improvement in the job market is continuing.
(A) so that it was the lowest in two years
(B) so that it was the lowest two-year rate
(C) to what would be the lowest in two years
(D) to a two-year low level
(E) to the lowest level in two years(答案)

我想问下，AB中的it，og说could refer to either decrease or unployment  但是，it不是特指么？指的应该是 the julydecrease in unemployment 整体啊怎会有歧义呢？