A farmer has 100 m of fencing to build a rectangular pasture along a straight riverbank, so no fencing is needed along the side bordering the river. If x meters denotes the length of each side perpendicular to the river, which of the following expresses the area A(x) of the pasture solely in terms of x?
a) A(x)=2x(100−x)
b) A(x)=x(100−x)
c) A(x)=x(100−2x)
d) A(x)=50x−x2