length of the crank

Question

    A quick return mechanism is shown below. The crank OS is drive at 2 rev/s in counter clockwise direction

 

If the quick rturn ratio is 1 : 2, then the length of the crank in mm is

      (a) 250                                                                (b) 250$\sqrt{3}$

      (c) 500                                                                (d) 500$\sqrt{3}$