A gear train is made up of five spur gears as shown in the figure. Gear $2$ is driver and gear $6$ is driven member. $N_2$, $N_3$, $N_4$, $N_5$ and $N_6$ represent number of teeth on gears $2$, $3$, $4$, $5$, and $6$ respectively. The gear(s) which act(s) as idler(s) is/are

1. Only $3$
2. Only $4$
3. Only $5$
4. Both $3$ and $5$