The volume (in cubic centimeters) of an open-top box formed by cutting congruent squares of side length x centimeters from each corner of a 20 cm×20 cm square sheet of cardboard and folding up the sides is V(x)=x(20−2x)2 for 0<x<10. For which value of x is the volume of the box greatest?
a) 4
b) 310
c) 2
d) 5