Points A=(3,9), B=(1,1), C=(5,3), and D=(a, b) lie in the first quadrant and are the vertices of quadrilateral A B C D. The quadrilateral formed by joining the midpoints of \overline{A B}, \overline{B C}, \overline{C D}, and \overline{D A} is a square. What is the sum of the coordinates of point D ?
Answer Choices
A. 7
B. 9
C. 10
D. 12
E. 16