$\text { If } 2 \cos ^{2} \theta-\sin \theta=1 \text { then the value of } \theta \text { is- }$

$\frac{\pi}{6} \text { and } \frac{5 \pi}{6}$

$\frac{3 \pi}{2}$

$\text { Both a and b }$

$\text { None of these }$