Which vehicle is unlawful to change gears while crossing the tracks of an uncontrolled railroad grade crossing?

Answer: C — Motor bus

Changing gears while crossing the tracks of an uncontrolled railroad grade crossing can pose serious safety risks, particularly for a motor bus. Buses are typically heavier and larger than other vehicles and have longer stopping distances. Additionally, they often transport multiple passengers, making the need for a smooth transit across railroad tracks even more critical. The concern with changing gears during a crossing is that it can cause a loss of power, which might lead to stalling on the tracks. In the case of a motor bus, the impact of such a stall is heightened since buses have specific safety regulations they must adhere to due to their size and the number of people on board. Therefore, this rule is particularly stringent for buses to ensure not only the safety of the driver but also of the occupants and other road users. By prohibiting gear changes while crossing, it aims to enhance the overall safety at these potentially hazardous crossings. Other vehicles may not be subject to the same restrictions to the same extent, given their different performance characteristics.