a) if f(x)= 1/e^x then f'(x)=?

b) if f'(x)= sin(x) then f(x)=?

c) If (0) = 0 then which one is a possible function:

1) f(x)=cos(x)?

2) f(x)=sin(x)?

3) 2^x?

4) e^x?