AMC 8 2011 Problem 20

Quadrilateral $ABCD$ is a trapezoid, $AD = 15$, $AB = 50$, $BC = 20$, and the altitude is $12$. What is the area of the trapezoid?

$\textbf{(A) }600 \textbf{(B) }650 \textbf{(C) }700 \textbf{(D) }750 \textbf{(E) }800$