(3) In the expansion of $\left(x^{2}-\frac{1}{x}\right)^{n}$, the constant term is 15, then the value of $n$ is ( ).
(A) 3
(B) 6
(C) 9
(D) 12
(3) B Hint: In the expansion of $\left(x^{2}-\frac{1}{x}\right)^{n}$, the constant term is 15, then $\mathrm{C}_{n}^{k}\left(x^{2}\right)^{k}\left(-\frac{1}{x}\right)^{n-k}=15$, where $2 k+k-n=0$, i.e., $k=\frac{n}{3}$, so $n$ should be divisible by 3, thus $\mathrm{C}_{n}^{\frac{n}{3}}=15$. When $n=3$, $\mathrm{C}_{3}^{1}=3 \neq 15$, when $n=6$, $\mathrm{C}_{6}^{2}=15$.