OG11-12 继续讨论

12. After gradual declension down to about 39 hours in 1970, the workweek in the United States has steadily increased to the point that the average worker now puts in an estimated 164 extra hours of paid labor a year.

A. After gradual declension down

B. Following a gradual declension down

C. After gradual declining down

D. After gradually declining

E. Following gradually declining

答案是D.没有疑问.

关于E, OG的解释是
    “Phrase must be introduced by a preposition(after),not a participle(following)”

But I think "after" should be conj. " After the workweek was gradually declining" 究竟为什么following 跟doing sth?