Isosceles trapezoid A B C D has parallel sides \overline{A D} and \overline{B C}, with B C<A D and A B=C D. There is a point P in the plane such that P A=1, P B=2, P C=3, and P D=4. What is \dfrac{B C}{A D} ?
Answer Choices
A. \dfrac{1}{4}
B. \dfrac{1}{3}
C. \dfrac{1}{2}
D. \dfrac{2}{3}
E. \dfrac{3}{4}