Braking hard on a wet mountain descent, a driver feels the brake pedal pulsing rapidly underfoot while the vehicle still responds to the steering. What is happening?

1. The brake fluid is boiling and steam bubbles are vibrating the pedal
2. The Anti-lock Braking System is releasing and reapplying brake pressure to stop the wheels locking, so steering control is retained
3. The handbrake cable has partially seized against the rear drums
4. A wheel bearing has failed and is transmitting vibration through the pedal