1. None of these are sentences, they are phrases. 2. As phrases, none of them sound weird. 3. C has been and is still studying math. :)

None are weird. Only "C" is clearly still studying now.

All 3 are correct. B and C are still studying at present. Thanks for asking!

To me, none of these sound weird. They all have there uses. "I studied math", "He has studied math", "She has been studying math". The one that might throw people off the most is the last one, but I do not feel that it is awkward given the right circumstance. "Has Sally been studying math? Yes, Sally has been studying math".

The answer is C