A short chain is usual attached to the back of a petrol tanker trailing behind it to ensure that the

• A petrol tanker is balanced on the wh eel
• B heat generated by friction in the engine can be conducted to the floor
• C charges generated by friction in the tanker can be conducted to the earth
• D the tanker moves slowly as the chain touches the floor
• E chain produces sound for the resonance of the tanker engine