I'm currently a grad student in a master's program in a biological science discipline. I'm starting to notice that I'm having some...issues with my peers/friends. We're sort of friends by default because there are only 4 (A, B, and C, plus me) American students in my program, and then there's the Chinese girl in the group because she's the only Chinese person in our year.

I'm not sure how to feel about these people. We had a test today, and met up on Monday and Wednesday to study together. Both times, it devolved into me explaining all the material that the test covered to them. To a point, I don't mind it, since explaining it helps me learn it. I should not, however, be a substitute for the professor. And to be honest, a lot of the stuff that I was explaining, they should have learned as an undergrad.

Friend A is really obnoxious about it--"OWLIE, SHOW ME HOW TO DRAW THE GRAPHS! CAN YOU EXPLAIN [BROAD CONCEPT] TO ME?" While she has her good points, she manages to act really stupid and immature at times (so people will explain things to her), and yet everyone in our class thinks she's the smart one because she's loud. (So they all go to her with questions!) I really don't mind explaining things to friends B and C. B tends to at least think about things before she asks me, or at least will admit she's tried and doesn't understand. C has no biology background (he was a chemical engineering major), so I don't mind explaining biology to him, because at least I don't have to tell him stuff that he should have learned in general chemistry.

I feel used. I lost my patience with A and B this morning while trying to explain something, so I feel guilty about it. I really don't want to break off being friends with A, because when she's not acting stupid or asking me something for the 20th time, she's a pretty good friend. (Also because then I really wouldn't have any friends in this class, since she and B and C are quite buddy-buddy.) What do I do?