For 0<r<9, a solid has identical rectangular cross sections of area A(r)=3r(9−r) square centimeters throughout its 5-centimeter length. The volume of the solid is therefore V(r)=45r(9−r) cubic centimeters. Which value of r maximizes the volume?
a) r=6
b) r=4.5
c) r=9
d) r=3