A 20 cm×30 cm sheet of cardboard is to be turned into an open-top box by cutting squares of side length x centimeters from each corner and folding up the sides. For 0<x<10, which value of x produces the box with the greatest volume?
a) x≈6.2 cm
b) x=5 cm
c) x≈3.9 cm
d) x=2 cm