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. $24$