The Tree in the Illustration Casts a Shadow $c = 6$

Multiple Choice

The tree in the illustration casts a shadow $c = 6$  feet long, while a person $a = 6$  feet tall casts a shadow $b = 4$  feet long. 
Find the height of the tree.

A) $\frac { 3 } { 12 }$
B) 15
C) $\frac { 1 } { 9 }$
D) 9
E) none of these