5. Let $\frac{x}{x^{2}-m x+1}=1$. Then the value of $\frac{x^{3}}{x^{6}-m^{2} x^{3}+1}$ is ( ).
(A) 1
(B) $\frac{1}{m^{3}+3}$
(C) $\frac{1}{3 m^{2}-2}$
(D) $\frac{1}{3 m^{2}+1}$
5. C (Hint: From the condition, we know $x \neq 0$, thus $\frac{x^{2}-m x+1}{x}$ $=1$, which means $x+\frac{1}{x}=m+1$, then transform the required expression accordingly.)