TIPS :Two -ing's in a row is a no-no.

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 every year. 

D) After graduay declining. 
E) Following gradualy declining. 

Manhattan Instructor > As a general rule, avoid short phrases that string together multiple -ing forms - that's always considered awkward and often introduces ambiguity. In this case, E presents us with a participle (following) followed closely by a gerund (declining). 

Following can be either a participle or a preposition. In preposition form, it would indeed have the same meaning as "after." In participle form, it would mean that the "declining" dictated that the workweek increase - as though the "declining" somehow told the workweek to increase. That last doesn't make sense. 

By contrast, the preposition "after" just introduces a temporal distinction, which is what we want here: one thing happened after another thing happened. Generally, when you want to make a temporal distinction, "after" is preferable to "following."

Author > One more quick question - 'Following' is a participle but not a preposition here because it is followed by an adverb ('gradually') and not the object of preposition? 

Manhattan Instructor > "Following" can be a preposition. If it is used as a preposition, though, it can only be paired with a NOUN or a PRONOUN (no gerunds). 

The adverb serves only to modify 'declining,' and, like other descriptive words, is irrelevant to the grammar of the sentence.