15. If $x+\frac{1}{x}=\sqrt{5}$, then the value of $x^{11}-7 x^{7}+x^{3}$ is ( ).
(A) -1
(B) 0
(C) 1
(D) 2
(E) $\sqrt{5}$
15. B.
$$
\begin{array}{l}
\text { Given } x+\frac{1}{x}=\sqrt{5} \Rightarrow x^{2}+\frac{1}{x^{2}}+2=5 \\
\Rightarrow x^{2}+\frac{1}{x^{2}}=3 \Rightarrow x^{4}+\frac{1}{x^{4}}+2=9 \\
\Rightarrow x^{4}+\frac{1}{x^{4}}=7 \text {. } \\
\text { Therefore } x^{11}-7 x^{7}+x^{3}=x^{7}\left(x^{4}+\frac{1}{x^{4}}-7\right)=0 .
\end{array}
$$