The funetion $f(x)=\frac{\log (\pi+x)}{\log (e+x)}$ is
(a) increasing on $[0, \infty)$
(b) decreasing on $[0, \infty)$
(c) increasing on $\left[0, \pi^{\prime} e\right)$ and decreasing on $[\pi e, \infty)$
(d) decreasing on $[0, \pi e)$ and increasing on $[\pi l e, \infty)$