What is the function of the 'Crankcase Ventilation System' (PCV) in an internal combustion engine?

1. To route blow-by gases from the crankcase back to the intake manifold to be burned
2. To blow fresh air into the crankcase to cool the lubricating oil
3. To vent combustible fuel vapors directly into the atmosphere to relieve pressure
4. To filter carbon deposits out of the oil pan sump