A flownet below a dam consists of $24$ equipotential drops and $7$ flow channels. The difference between the upstream the upstream and downstream water levels is $6$ m. The length of the flow line adjacent to the toe of the dam at exit is $1$ m. The specific gravity and void ratio of the soil below the dam are $2.70$ and $0.70$, respectively. The factor of safety against piping is

1. $1.67$
2. $2.5$
3. $3.4$
4. $4$