If $f , g$ are real-valued differentiable functions on the real line $\mathbb { R }$ such that $f ( g ( x ) ) = x$ and $f ^ { \prime } ( x ) = 1 + ( f ( x ) ) ^ { 2 }$, then $g ^ { \prime } ( x )$ equals\\
(A) $\frac { 1 } { 1 + x ^ { 2 } }$\\
(B) $1 + x ^ { 2 }$\\
(C) $\frac { 1 } { 1 + x ^ { 4 } }$\\
(D) $1 + x ^ { 4 }$.