Which motor vehicle is unlawful to change gears while crossing an uncontrolled railroad grade?

Answer: B — Motor bus

When dealing with railroad grade crossings, safety is of utmost importance. Changing gears while crossing an uncontrolled railroad grade can increase the risk of stalling, which could result in a vehicle being stranded on the tracks. Motor buses, due to their size, weight, and the nature of their operation, are particularly sensitive to changes in power and momentum. Changing gears while crossing can disrupt their balance and acceleration, potentially leading to a dangerous situation on the tracks. Therefore, regulations prohibit motor buses from changing gears during these crossings to ensure that they can maintain a constant speed and clear the area quickly and safely. In contrast, while trucks, vans, and passenger cars may also have rules regarding their movements at railroad crossings, the specific prohibition for motor buses stems from their operational characteristics and the higher risk they pose if stopped on the tracks.