One of the factors of $x^4+2x^2+9$ is: 
$\textbf{(A)}\ x^2+3\qquad\textbf{(B)}\ x+1\qquad\textbf{(C)}\ x^2-3\qquad\textbf{(D)}\ x^2-2x-3\qquad\textbf{(E)}\ \text{none of these}$
We can test each of the answer choices by using polynomial division. 
$x^2 + 3$ leaves behind a remainder, and so does $x^2 - 3$.
In addition, $x + 1$ also fails the test, and that takes down $x^2 - 2x - 3$, which can be expressed as $(x + 1)(x - 3)$. That leaves $\boxed{(\textbf{E})}$