In the diagram, square P Q R S has side length 2. Points M and N are the midpoints of S R and R Q, respectively. The value of \cos (\angle M P N) is
Answer Choices
A. \frac{4}{5}
B. \frac{\sqrt{2}}{2}
C. \frac{\sqrt{5}}{3}
D. \frac{1}{3}
E. \frac{\sqrt{3}}{2}