1. Let $a=(x+1)(x+2)(x+3)(x+4)$, $b=(x-4)(x-3)(x-2)(x-1)$. Then $a-b$ equals ( ).
(A) $20 x^{3}+50 x$
(B) $2 x^{3}+5 x$
(C) $20 x^{4}+100 x^{2}$
(D) $20 x^{3}+100 x$
\begin{array}{l}-.1 .(\mathrm{D}) . \\ \because a=(x+1)(x+4) \cdot(x+2)(x+3) \\ =\left(x^{2}+5 x+5\right)^{2}-1 \text {. } \\ b=(x-4)(x-1) \cdot(x-3)(x-2) \\ =\left(x^{2}-5 x+5\right)^{2}-1 \text {, } \\ \therefore a-b=\left(x^{2}+5 x+5\right)^{2}-\left(x^{2}-5 x+5\right)^{2} \\ =20 x^{3}+100 x \text {. } \\\end{array}