设 $\alpha_1=\sqrt{1+\tan x}-\sqrt{1+\sin x}, \alpha_2=\int_0^{x^4} \frac{1}{\sqrt{1-t^2}} d t, \alpha_3=\int_0^x d u \int_0^{u^2} \arctan t d t$. 当 $x \rightarrow 0$ 时, 以上 3 个无穷小量按照从低阶到高阶的排序是（）
A
$\alpha_1, \alpha_2, \alpha_3$.
B
$\alpha_1, \alpha_3, \alpha_2$.
C
$\alpha_2, \alpha_1, \alpha_3$.
D
$\alpha_3, \alpha_1, \alpha_2$.
E
F