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)