21. Given that $f(x)$ is a real function, and $x \neq 0$, and $f(x)$ satisfies $f\left(\frac{1}{x}\right)+\frac{1}{x} f(-x)=2 x$. Then the value of $f(2)$ is $(\quad$.
(A) 2.5
(B) 3
(C) 3.5
(D) 4
(E) 4.5
21.E.

Let $u=f(2), v=f\left(-\frac{1}{2}\right)$. Set $x=\frac{1}{2}, x=$
-2, to get $u+2 v=1, v-\frac{1}{2} u=-4$. Solving these, we get $u=\frac{9}{2}$. Therefore, $f(2)=u=\frac{9}{2}$. The function $f(x)=\frac{x^{3}+1}{x}$ satisfies these conditions.