og11 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.

(D) After gradually declining

(E) Following gradually declining

正确选项是D. 但我不太明白E错在哪里。OG的解释是 Phrase must be introduced by a preposition (after), not a participle (following).

请问这是通用的规则，还是只是在这道题里after 优于 following?