Which two answers are correct?

Here's the basic question:

We have an integral, that of which we do not know, where two out of three **students got the correct answer. Which two students are right?**

a. $\displaystyle sin^2x+C$

b. $\displaystyle -cos^2x+C$

c. $\displaystyle -sin^2x+C$

My logic is that one of the $\displaystyle (-)sin^2x+C $ is right, meaning the integral might be $\displaystyle \int2sin(x)cos(x)$, but I'm not sure how one could get a $\displaystyle -cos^2x+C$ out of that.

Any help is appreciated.