Choose the correct statement with regard to inspection of the engine during driving.

A. It is normal for knocking to occur when the accelerator pedal is depressed and the vehicle accelerates

B. Knocking and pinging occur as a result of variations in fuel quality and do not constitute an engine abnormality

C. Poor fuel quality leads to failure of the engine to stop immediately when the key is moved to the OFF position, consequently, no maintenance need to be carried out in such a case

D. The engine's idling speed will increase by a light degree immediately after a cold engine has been started, it will drop as the engine warms up, and it will eventually reach the standard level

Answer: Option D