Which of the following functions satisfies f(x+y)=f(x)+f(y)f(x+y) = f(x) + f(y)f(x+y)=f(x)+f(y) for all real x,yx, yx,y?
f(x)=x2f(x) = x^2f(x)=x2
f(x)=3xf(x) = 3xf(x)=3x
f(x)=1xf(x) = \frac{1}{x}f(x)=x1
f(x)=x+1f(x) = x+1f(x)=x+1