James is writing a coordinate proof involving a parallelogram. Knowing that the opposite sides of a parallelogram are congruent, James places his parallelogram on the coordinate plane such that one vertex is at the origin and one side lies along the x-axis.

What coordinates should he assign to the fourth vertex of the parallelogram?

A. (2a, a)

B. (2a, a2)

C. (a2, 0)

D. (2a, 2a)