3. Given $x \in\left(0, \frac{\pi}{2}\right)$, then the integer part of the number $m=3^{\cos ^{2} x}+3^{\sin ^{4} x}$ is
A. 2
B. 3
C. 4
D. Cannot be determined
3. $\mathrm{B}$

Let $t=\sin ^{2} x$, then $0<t<1$, so $m=3^{t}+3^{1-t}=3^{t}+\frac{3}{3^{t}}, 3^{t} \in(1,3)$, hence $2 \sqrt{3} \leqslant m<4$.