\triangle{ABC} is an isosceles triangle in which AB=AC=10 and BC=12. The points S and R are on BC such that BS:SR:RC=1:2:1. The midpoints of AB and AC are P and Q respectively. Perpendiculars are drawn from P and R to SQ meeting at M and N respectively. The length of MN is
Answer Choices
A. \frac{9}{\sqrt{13}}
B. \frac{10}{\sqrt{13}}
C. \frac{11}{\sqrt{13}}
D. \frac{12}{\sqrt{13}}
E. \frac52