How does riding at high altitudes (e.g., in Baguio City) affect a carbureted motorcycle's engine performance, and how do you correct it?

1. The lower air density causes the engine to run rich; you correct it by installing a smaller main jet (leaning the mixture).
2. The lower air density causes the engine to run lean; you correct it by installing a larger main jet.
3. High altitude increases oxygen levels, requiring a larger main jet to prevent engine knocking.
4. Altitude only affects tire pressure and has no impact on carburetor air-fuel ratios.