If $f(x)+2 f\left(\frac{1}{x}\right)=3 x, x \neq 0$, and $\mathrm{S}=\{x \in \mathbf{R}: f(x)=f(-x)\}$; then $\mathrm{S}:$

Question

is an empty set.

contains exactly one element.

contains exactly two elements.

contains more than two elements.

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