Well: the following six words can act as both a sub-conjunction and a preposition. If it is a preposition, then a noun follows. If it is acting as sub-conjunction, an IC with a full-fledged verb follows. Those six words are
1. After 2 Before 3. Until 4 Since 5. For 6. As
Let us see each of them:
After: I slept after my dinner (preposition) – dinner is a noun; it can also be a gerund as in I slept after reading
I slept after I felt tired: here after is a sub-conjunction followed by a verbed clause.
Before: I jogged before my bath – preposition
I jogged before I took my bath – conjunction
As: I am correct in my duties as a father --- preposition
I am correct in my duties as a father is required to do - Conjunction
Since: I slept for a good time since your arrival - preposition
I slept for a long time , since you arrived very late
Until: I slept late until 9 AM – preposition
I slept late until you arrived – conjunction
For: I slept for 12 hours- preposition
I slept the whole day, for I felt tired. Conjunction ( for used in the meaning of because)
So we have to see, whether the said word is used as a preposition or a conjunction, before taking a blanket decision
E is still wrong in the given case for other reasons.
HTH
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