Which one of the following links should be fixed for the resulting mechanism to be a double crank mechanism?

Question

ABCD is a mechanism with link lengths     AB = 200, BC = 300, CD = 400 and DA = 350. Which one of the following links should be fixed for the resulting mechanism to be a double crank mechanism? (All length are in mm)                 

(a) AB                                   (b) BC

(c) CD                                     (d) DA