13. If $f ( x ) = ( x 2 - 1 ) / ( x 2 + 1 )$, for every real number $x$, then the minimum value of $f$ : (A) does not exist because $f$ is unbounded. (B) is not attained even though $f$ is bounded (C) is equal to 1 (D) is equal to - 1
13. If $f ( x ) = ( x 2 - 1 ) / ( x 2 + 1 )$, for every real number $x$, then the minimum value of $f$ :\\
(A) does not exist because $f$ is unbounded.\\
(B) is not attained even though $f$ is bounded\\
(C) is equal to 1\\
(D) is equal to - 1\\