3. The smallest positive period of the function $y=\cos ^{4} x+\sin ^{2} x$ is ( ).
(A) $\frac{\pi}{4}$
(B) $\frac{\pi}{2}$
(C) $\pi$
(D) $2 \pi$
3. B.
$$
\begin{array}{l}
\text { Since } y=\cos ^{4} x+\sin ^{2} x \\
=\cos ^{2} x\left(1-\sin ^{2} x\right)+\sin ^{2} x=1-\sin ^{2} x \cdot \cos ^{2} x \\
=1-\frac{1}{4} \sin ^{2} 2 x=\frac{7}{8}+\frac{1}{8} \cos 4 x,
\end{array}
$$

Therefore, the smallest positive period of the function $y=\cos ^{4} x+\sin ^{2} x$ is $\frac{\pi}{2}$.