What is the primary function of the 'kneeling' (Kneeling-Funktion) system on modern low-floor city buses?

1. To raise the rear axle when driving over steep speed bumps
2. To lower the total vehicle roof height when entering low maintenance tunnels
3. To lower the passenger entrance side of the bus towards the kerb to facilitate boarding for elderly, disabled passengers, and strollers
4. To tilt the bus inwards during high-speed cornering