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
- $8.5$
- $7.0$
- $6.5$
- $5.5$