请教PREP1 31

31.
            (26141-!-item-!-188;#058&002444)

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

解释说B和D选项改变了原句的意思，想问一下怎么改变了啊？谢谢大家。

the lowest level in two years 两年内的最低水平（比如99年初到00年末之间的一个点）

lowest two-year rate两年（这个时间段的）的最低水平（99年初到00年末这一个时间段与00年初到01年末这个时间段对比）