At a particular value of y, a horizontal slice of a planar region has left boundary at x=xL(y) and right boundary at x=xR(y). Which differential expression represents the area of this slice to be used in a definite integral with respect to y?
a) (xR(y)+xL(y))dy
b) dyxR(y)−xL(y)
c) (xR(y)−xL(y))dy
d) (xL(y)−xR(y))dy