Under the current PANS-ATM aerodrome-control rule, when may a tower issue takeoff clearance behind a preceding departure on the same runway?

1. When there is reasonable assurance that the prescribed runway separation will exist when the following aircraft begins takeoff, with wake-turbulence and any authorized reduced-runway-separation rules also satisfied.
2. As soon as the preceding aircraft starts its takeoff roll.
3. When the preceding aircraft reaches 50 knots groundspeed.
4. Whenever the runway visibility exceeds 10 km.