Determining which functions are one to one 
So I know for sure that $x^3$ is one to one, being that the entire span of real number $y$'s can be found by some x and it passes the vertical line test. Graphically, its pretty clear. I know that $x^2$ is also one to one though not onto, since no negative $y$'s can be obtained. I don't have the same graphical intuition for options B, D, and E. How would I go about making that determiniation? 

Hints: For (B), consider $f(x,x,x)$.
For (D), consider $f(x,-x)$ 
For (E), find $x$ and $y$ given $x+y = a$ and $x-y = b$.
