Thomas is building rectangular boxes. The table shows the area in square feet, f(x), of the bottom of a box when one side of the box is x feet long.

x 1 2 3 4 5
f(x) 2 8 18 32 50

A f(x)= x^2+2x
B f(x)= x^2/2
C f(x)=2x^2
D f(x)=2x