$29 \cdot 39 x^{4}+4$ is a factor of
(A) $x^{2}+2$.
(B) $x+1$.
(C) $x^{2}-2 x+2$.
(D) $x^{2}-4$.
(E) None of the above.
(4th American High School Mathematics Examination, 1953)
[Solution] $x^{4}+4=x^{4}+4 x^{2}+4-4 x^{2}=\left(x^{2}+2\right)^{2}-4 x^{2}$ $=\left(x^{2}+2 x+2\right)\left(x^{2}-2 x+2\right)$.

Therefore, the answer is $(C)$.