请教一道语法题

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，E呢？after和following怎么区别？