Multiply Assume That All Variables Represent Natural Numbers $\left( x ^ { a } + 7 \right) \left( x ^ { 2 a } + 3 \right)$

Multiple Choice

Multiply. Assume that all variables represent natural numbers.
- $\left( x ^ { a } + 7 \right) \left( x ^ { 2 a } + 3 \right)$

A) $x ^ { 3 a } + 10 x ^ { a } + 21$
B) $x ^ { 3 a } + 10 x ^ { 2 a } + 21$
C) $x ^ { 3 a } + 7 x ^ { 2 a } + 3 x ^ { a } + 21$
D) $x ^ { 2 a ^ { 2 } } + 7 x ^ { 2 a } + 3 x ^ { a } + 21$