若 $M=\int_{\frac{\pi}{2}}^{\frac{\pi}{2}} \frac{\sin x}{1+x^2} \cos ^4 x d x, \quad P=\int_{\frac{\pi}{2}}^{\frac{\pi}{2}}\left(x^2 \sin ^3 x-\cos ^4 x\right) d x, \quad N=$ $\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}}\left(\sin ^3 x+\cos ^4 x\right) d x$, 则 $(\quad)$