The radii of the front wheel and the rear wheel of a bike are 14 cm and 21 cm respectively. Rahul puts a red mark on the point of contact of each of the wheels with the ground when the bike is stationary. Once the bike start moving, then after what distance will the two red marks touch the ground again simultaneously? (a) 42 cm      (b) 84 cm    (c) 264 cm   (d) 294 cm