Multiply and Then, If Possible, Simplify by Factoring $\sqrt{7} \sqrt{3 x-7}$

Multiple Choice

Multiply and then, if possible, simplify by factoring.
- $\sqrt{7} \sqrt{3 x-7}$

A) $\sqrt{21 x-7}$
B) $21 x+49$
C) $\sqrt{21 x-49}$
D) $\sqrt{21 x+49}$