Determine Which of the Following Polynomials Is Not Reducible $x ^ { 2 } + 18$

Multiple Choice

Determine which of the following polynomials is not reducible.

A) $x ^ { 2 } + 18$
B) $x ^ { 2 } - 4$
C) $x ^ { 2 } - 1$
D) $x ^ { 2 } - 21$
E) $x ^ { 2 } - 18$