Square P Q R S lies in the first quadrant. Points (3,0),(5,0),(7,0), and (13,0) lie on lines S P, R Q, P Q, and S R, respectively. What is the sum of the coordinates of the center of the square P Q R S ?
Answer Choices
A. 6
B. 6.2
C. 6.4
D. 6.6
E. 6.8