AMC 12A 2002 Problem 18

Let C_{1} and C_{2} be circles defined by

(x-10)^{2}+y^{2}=36

and

(x+15)^{2}+y^{2}=81

respectively. What is the length of the shortest line segment \overline{P Q} that is tangent to C_{1} at P and to C_{2} at Q ?

Answer Choices
A. 15
B. 18
C. 20
D. 21
E. 24