Thread: Finding the possible values of f(0)

I'm taking the placement test of college and this is one of the study questions.
How do I figure this out?

If the function f satisfies the equation f(x+y)=f(x)+f(y) for every pair of real numbers x and y, what are the possible values of f(0)?
A. any real number
B. Any positive real number
C. 0 and 1 only
D. 1 only
E. 0 only

f(0) = f(0 + 0) = f(0) + f(0), so...