gear and gear train

Question

1. A gear train is made up of five spur gears as shown in the figure. Ger 2 is driver and gear 6 is driven member. N₂, N₃, N₄, N₅ and N₆ represent number of teeth on gears 2,3,4,5 and 6 respectively. The gear(s) which act(s) as idler(s) is/are

     (a) Only 3                                                                 (b) Only 4

      (c) Only 5                                                                 (d) both 3 and 5