At which point does the responsibility for control of an aircraft transfer from one air traffic control unit to another?

1. Exactly at the common control area boundary, unless a letter of agreement (LOA) specifies otherwise.
2. When the aircraft is first detected on the receiving unit's radar screen.
3. Only after the pilot establishes two-way radio communication with the receiving unit.
4. Five minutes prior to the aircraft's estimated time over the boundary facility.