The lateral surface (sides but not the two ends) of a 10-meter-long box is to be constructed from 120 square meters of sheet metal. The box has a rectangular cross section of width w meters and height h meters. If all the available sheet metal is used, what value of w maximizes the volume of the box?
a) w=4 m
b) w=3 m
c) w≈2.6 m
d) w=6 m