An open-top box with a square base is to be constructed to hold a fixed volume V. The material for the base costs three times as much per square unit as the material for the sides. If the base edge length is x and the height is h, which of the following ratios of height to base edge minimizes the total cost of materials?
a) xh=32
b) xh=23
c) xh=21
d) xh=3