A square piece of cardboard measuring 20 cm on a side is to be made into an open box by cutting identical squares of side length x from each corner and folding up the sides. Which expression gives the volume V(x), in cm3, of the box in terms of x?
a) x(20−x)2
b) x(20−2x)2
c) 400x−4x3
d) x(20−2x)(20−x)