Transform the rectangular equation $x^2=31y$ into a polar equation expressing $r$ in terms of $\theta$.

• A. $r=\frac{31}{{\cos }^2\theta }$
• B. $r=\frac{31\sin \theta }{{\cos }^2\theta }$
• C. $r=\frac{31\cos \theta }{{\sin }^2\theta }$
• D. $r=\frac{31\sin \theta }{\cos \theta }$

Answer: (B)