AMC 8 2016 Problem 2

In rectangle $ABCD$ , $AB=6$ and $AD=8$ . Point $M$ is the midpoint of $\overline{AD}$ . What is the area of $\triangle AMC$ ? $\textbf{(A) }12 \textbf{(B) }15 \textbf{(C) }18 \textbf{(D) }20 \textbf{(E) }24$