An isolated $3-h$ rainfall event on a small catchment produces a hydrograph peak and point of inflection on the falling limb of the hydrograph at $7$ hours and $8.5$ hours respectively, after the start of the rainfall. Assuming, no losses and no base flow contribution, the time of concentration (in hours) for the catchment is approximately

1. $8.5$
2. $7.0$
3. $6.5$
4. $5.5$