The operation of vehicles is prohibited if:

Explanation

Both the threaded joints of the steering not being tightened or secured and the absence of fire extinguishers on the bus.

This question places a mechanical defect in the steering system alongside an item of mandatory equipment; both are on the list of prohibiting defects.

The threaded joints of the steering system must be tightened and secured in the prescribed manner — to a specified torque and with a self-locking nut, split pin or locking wire. The reason is vibration: the steering constantly takes shocks from the road surface, and a bolt that is merely tightened but not secured works loose over time. The end result is a steering rod or ball joint separating while the vehicle is moving; that is, a complete and sudden loss of directional control with no prior warning. No other system in the vehicle can take the steering's place.

A fire extinguisher on a bus has an equally clear logic: fire in a vehicle carrying dozens of passengers with one or two exits becomes a disaster within minutes. The extinguisher is for those first few minutes — while the fire is still limited and containable and the fire brigade's arrival will take several times as long.

Worth memorising: for a bus, keep three items of mandatory equipment together — the first-aid kit, the fire extinguisher and the emergency stop sign. The absence of any one of them prohibits movement on its own.

• Correct but incomplete; untightened threaded joints in the steering really do prohibit operation, but the option drops the bus fire extinguisher item. • The absence of a fire extinguisher on a bus is independently prohibiting too; choosing it alone makes the answer incomplete.

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