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

Question

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