What is the primary risk of carrying unsecured heavy luggage or loose cargo on the rear parcel shelf of a passenger car?

1. In a frontal collision, loose objects fly forward with lethal projectile force, striking front-seat occupants in the head and neck
2. The vehicle fuel tank will pressurize and fail
3. The rear windscreen will automatically shatter during normal cruising
4. The vehicle brake pads will wear out twice as fast