(7) The constant term in the binomial expansion of $\left(x-\frac{1}{x^{6}}\right)^{2009}$ is ( ).
(A) $\mathrm{C}_{2009}^{286}$
(B) $\mathrm{C}_{207}^{287}$
(C) $-\mathrm{C}_{2009}^{286}$
(D) $-\mathrm{C}_{2009}^{287}$
(7) D Hint: Since
$$
\left(x-\frac{1}{x^{6}}\right)^{2009}=\sum_{k=0}^{2009} \mathrm{C}_{2009}^{k} x^{2009-k}\left(-x^{-6}\right)^{k},
$$

for a constant term to appear, it must satisfy
$$
2009-k-6 k=0 \Rightarrow k=287 \text {. }
$$