How many fire extinguishers must be present in a bus, and where must they be secured if places for their mounting are not provided for by the design?

• 1One fire extinguisher, which must be secured in the driver's cab.
• 2Two fire extinguishers, which must be secured in the driver's cab.
• 3Two fire extinguishers, one of which must be secured in the driver's cab and the other in the passenger compartment.
• 4Three fire extinguishers, one of which must be secured in the driver's cab, the second in the passenger compartment, and the third in the luggage compartment.