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

( A ) contains exactly one element
( B ) is an empty set.
( C ) contains more than two elements.
( D ) contains exactly two elements.