What is the purpose of the small drain hole in the pitot tube?

1. 1To measure static pressure inside the chamber
2. 2To let moisture drain from the system
3. 3To cool the pitot heater element in flight
4. 4To let excess ram air escape from the tube

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 8: Flight Instruments — Pitot-Static Flight Instruments — Impact Pressure Chamber and Lines, page 204