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