2. Let $\alpha$ be an acute angle. If
$$
\begin{array}{l}
M=\frac{\sin ^{4} \alpha+\cos ^{4} \alpha}{\sin ^{6} \alpha+\cos ^{6} \alpha}, \\
N=\frac{\sin ^{4} \alpha+\cos ^{4} \alpha}{\sqrt{\sin ^{4} \alpha+\cos ^{4} \alpha}}, \\
P=\frac{\sqrt{\sin ^{4} \alpha+\cos ^{4} \alpha}}{\sin ^{6} \alpha+\cos ^{6} \alpha},
\end{array}
$$

then the size relationship of $M$, $N$, and $P$ is ( ).
(A) $M<N<P$
(B) $N<P<M$
(C) $P<M<N$
(D) $N<M<P$
2. D.

Let $\alpha=45^{\circ}$. Then
$$
M=2, N=\frac{\sqrt{2}}{2}, P=2 \sqrt{2} \text {. }
$$

Thus $N<M<P$.