On a road, the speed – density relationship of a traffic stream is given by $u=70-0.7k$ (where speed, $u$, is in $\text{km/h}$ and density, $k$ is in $\text{veh/km}$). At the capacity condition, the average time headway will be

1. $0.5 \:s$
2. $1.0 \:s$
3. $1.6 \:s$
4. $2.1 \:s$