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.
- $\mathrm{a}=1, \mathrm{c}=21$

A) $\mathrm{b}=\sqrt{20} ; \mathrm{b} \approx 4.472$
B) $b=20$
C) $b=\sqrt{440} ; b \approx 20.976$
D) $b=\sqrt{442} ; b \approx 21.024$