设 $I_1=\int_0^{\frac{\pi}{2}} \frac{\cos x}{1+x^2} \mathrm{~d} x, I_2=\int_0^{\frac{\pi}{2}} \frac{\sin x}{1+x^2} \mathrm{~d} x, I_3=\int_0^{\frac{\pi}{2}} \frac{\sin x}{(1+x)^2} \mathrm{~d} x$, 则
A. $I_1>I_2>I_3$.
B. $I_3>I_2>I_1$.
C. $I_2>I_1>I_3$.
D. $I_2>I_3>I_1$.