A rectangular yard contains two flower beds in the shape of congruent isosceles right triangles. The remainder of the yard has a trapezoidal shape, as shown. The parallel sides of the trapezoid have lengths 15 and 25 meters. What fraction of the yard is occupied by the flower beds?
Answer Choices
A. \dfrac{1}{8}
B. \dfrac{1}{6}
C. \dfrac{1}{5}
D. \dfrac{1}{4}
E. \dfrac{1}{3}