Hi,
Again the following question was taken from a test but I can't understand why the testee has given "when" and "as" together among the choices. What do you think the answer is. I don't see a sound reason to prefere "a" over "c", I agree with you. Actually, I prefer 'as', since it gives a sense of duration.
Someone knocked on the door --- the students were studying. (I hope the omission of 'on' is just your typo!
)
a) when OK
b) since
c) as OK
d) which
I suppose the testee would consider it frivolous if I suggested that both B and D are also grammatically correct, and that I could invent a scenario in which they were semantically correct. My feeling is that such multiple choice tests should only contain one answer that is in any way possible. Otherwise, it becomes a matter of 'Choose which one you think is the most likely'.
Best wishes, Clive