Let $f(x)$ be a real valued function defined for all $x \geq 1$, satisfying $f(1)=1$ and $f^{\prime}(x)=\frac{1}{x^{2}+(f(x))^{2}}$; then $\lim _{x \rightarrow \infty} f(x)$
(a) doesn't exist
(b) exists and less than $\frac{\pi}{4}$
(c) exists and less than $1+\frac{\pi}{x}$
(d) exists and equal to 0