A gear train with five gears is shown in the figure below. The number of teeth on each gear is $\mathrm{N}_{1}, \mathrm{~N}_{2}, \mathrm{~N}_{3}, \mathrm{~N}_{4}$, and $\mathrm{N}_{5}$. The idler gear in this gear train is

- gear $4$
- gear $2$
- gear $3$
- gear $1$