$A$ boat crosses a river from one bank $A$ to another bank $B$ which is opposite. The distance between them is $D$. The speed of water is $V_W$ and that of the boat relative to water is $V_B$. If $V_B = 2V_W$, the time taken by the boat to cross the river directly along $AB$ is $(\sin 30^\circ = 1/2, \cos 30^\circ = \sqrt{3}/2)$.

• A
  $\frac{D}{V_B\sqrt{2}}$
• B
  $\frac{D\sqrt{2}}{V_B}$
• C
  $\frac{2D}{V_B\sqrt{3}}$
• D
  $\frac{\sqrt{3}D}{2V_B}$