In Young's double slit experiment, one of the slit is wider than other, so that the amplitude of the light from one slit is double of that from other slit. If $I_m$ be the maximum intensity, the resultant intensity $I$ when they interfere at phase difference $\phi$ is given by

Question

( A ) $\frac{I_m}{9} (1 + 8 \cos^2 \frac{\phi}{2})$
( B ) $\frac{I_m}{5} (1 + 4 \cos^2 \frac{\phi}{2})$
( C ) $\frac{I_m}{3} (1 + 2 \cos^2 \frac{\phi}{2})$
( D ) $\frac{I_m}{9} (4 + 5 \cos^2 \phi)$