Points A \equal{} (3,9), B \equal{} (1,1), C \equal{} (5,3), and D \equal{} (a,b) lie in the first quadrant and are the vertices of quadrilateral ABCD. The quadrilateral formed by joining the midpoints of AB,BC,CD, and DA is a square. What is the sum of the coordinates of point D?
<spanclass=′latex−bold′>(A)</span> 7<spanclass=′latex−bold′>(B)</span> 9<spanclass=′latex−bold′>(C)</span> 10<spanclass=′latex−bold′>(D)</span> 12<spanclass=′latex−bold′>(E)</span> 16 analytic geometryrotationAMC