Factor the Following Trinomial Assuming That All Variables Appearing as Exponents

Multiple Choice

Factor the following trinomial assuming that all variables appearing as exponents represent positive integers. $x ^ { 2 a } + 10 x ^ { a } + 21$

A) $\left( x ^ { a } - 7 \right) \left( x ^ { a } + 3 \right)$
B) $\left( x ^ { a } + 7 \right) \left( x ^ { a } - 3 \right)$
C) $\left( x ^ { a } + 7 \right) \left( x ^ { a } + 3 \right)$
D) $\left( x ^ { a } - 7 \right) \left( x ^ { a } - 3 \right)$