5. Given the function $f(x)=(\cos \theta) x^{2}-(4 \sin \theta) x+6$ is greater than 0 for any real number $x$, and $\theta$ is an interior angle of a triangle, then the range of $\theta$ is
A. $(0, \pi)$
B. $\left(0, \frac{\pi}{2}\right)$
C. $\left(0, \frac{\pi}{3}\right)$
D. $\left(0, \frac{\pi}{6}\right)$
5. C According to the problem, $\cos \theta>0, \Delta<0$.