A rectangular garden is to be constructed against a straight wall, so fencing is required only for the two sides perpendicular to the wall and the side opposite the wall. The gardener has 80 m of fencing. What value of x, the length (in meters) of each side perpendicular to the wall, maximizes the enclosed area?
a) 40 m
b) 26.7 m
c) 20 m
d) 13.3 m