According to some economists, the July decrease 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

请问一下这一句的主语是什么？