AMC 10 2001 Problem 24

In trapezoid A B C D, \overline{A B} and \overline{C D} are perpendicular to \overline{A D}, with A B+C D= B C, A B<C D, and A D=7. What is A B \cdot C D ?

Answer Choices
A. 12
B. 12.25
C. 12.5
D. 12.75
E. 13