Find the Length of the Third Side of the Right $c$

Multiple Choice

Find the length of the third side of the right triangle. Assume that $c$ represents the length of the hypotenuse. Give an exact answer and, if appropriate, an approximation to three decimal places.
- $b=20, c=25$

A) $a=225$
B) $a=5$
C) $a=45$
D) $a=15$