A solid has rectangular cross-sections perpendicular to the x-axis. At a particular value of x, the base of the rectangle in the xy-plane extends from y=2 to y=5, and the rectangle’s height at that x is given by h(x)=3x. Which of the following expressions gives the area A(x) of that cross-section?
a) A(x)=3x+5−2
b) A(x)=21x
c) A(x)=3x(5−2)
d) A(x)=21(3x)(5−2)