For rectangles with the same area, the length varies inversely to the width. Henry is trying to decide how to plan the dimensions of his new rectangular swimming pool. He is planning on his pool being 200 square feet. One option would be to have a length of 20 ft and a width of 10ft. Write an equation to model this scenario.

A: 10=a/20
B: 20=a/10
C: 10=20/a
D: 20= 10/a