Triangle A B C has side-lengths A B=12, B C=24, and A C=18. The line through the incenter of \triangle A B C parallel to \overline{B C} intersects \overline{A B} at M and \overline{A C} at N. What is the perimeter of \triangle A M N ?
Answer Choices
A. 27
B. 30
C. 33
D. 36
E. 42