Data about vehicles needs to be stored. For each vehicle, the unique identification number, make, model, and year of manufacture need to be recorded. All of the vehicles are either a car, truck, or van. If the vehicle is a truck then the towing capacity, seating capacity, and payload capacity must be recorded. If the vehicle is a van, then the seating capacity must be recorded. If the vehicle is a car, then the seating capacity should also be recorded. Using the identification number as the primary key and after a proper subtype discriminator is created, how many attributes would be in the supertype based on the data requirements described?