What is the function of the reed valve in a standard crankcase-compression two-stroke motorcycle engine?

1. It acts as a one-way check valve that allows the fuel-air mixture to enter the crankcase from the carburetor, but prevents it from blowing back out when the piston descends.
2. It regulates the engine oil pressure in the top end of the cylinder block.
3. It controls the ignition spark timing relative to the piston position.
4. It allows exhaust gas to recirculate into the intake tract to reduce emissions.