"Excel at something" vs. "excel in something"

I've come across a question while writing an exam

Roger really excelled ___ sports

A) at
B) on
C) in
D) for

My first thought was 'in', later I remembered using 'at' also. I've searched for this in a few forums, there they say both 'excel in' and 'excel at' are correct expressions and could not really make out any differences.

Now my problem is that I'm an ESL and the above question doesn't give options like 'both a and c correct'. I'm not sure what option to pick for these kind of questions.

So, what is the correct option for above question?. Answers from experts are much appreciated. If both are correct what is the intention of question paper setter?

While 'excel at' and 'excel in' are generally interchangeable, I do think there is a subtle distinction.

excel at seems to apply better to specific activities.

He excels at service returns in badminton.

excel in seems to apply better to more general categories or things that cover many activities.

She excels in school.

To me, "She excels at school" sounds thoroughly ungrammatical and would not be uttered by a native speaker. "He excels in service returns in badminton" also sounds a little bit strange but other native speakers than me might use it.

I believe the intended correct answer is "in". You are good "at" something, but you excel "in" a sport or an activity.

Possibly it is also acceptable to say that someone excels "at" something, but "in" should be more correct. As in:

Little Johnny excels in debate class. He also excels in sports and in many other activities.

Note that in the above example it might sound OK to say "excels at sports" or "excels at many other activities"-- but "excels at debate class" is clearly wrong.

So I would argue that in is the true, correct preposition to be used with "excel", and at is a late-comer that has slipped into usage because it is applicable to many (but not all) of the things that one can excel in and therefore sounds OK when used in those phrases.

Update

Merriam Webster online (http://www.merriam-webster.com/dictionary/excel) gives examples of usage with both at and in:

excel in sports

excelled at lipreading

As they use 'in' in the phrase with 'sports', I maintain my position that in is more correct, at least in this specific case.

We use AT when we speak about sports, activities: excel at football.

And we use IN when we speak about academic subjects: excel in maths.

In Britain 'excel at' and 'excel in' are to all intents interchangeable. I cannot think of any example of one where you couldn't with equal justification also use the other. Can anyone?

Responder's own edit: see @ virmajor reply. They may be on to something here.