When can a landing aircraft be cleared to land on an active runway by the Tower controller?

1. When there is reasonable assurance that the runway will be completely clear of all preceding landing, departing, and crossing aircraft and vehicles before the landing aircraft crosses the runway threshold.
2. Even if a tractor is crossing the runway threshold.
3. Only when the airport is completely shut down to other flights.
4. When the pilot signs a waiver on the radio.