Why should you allow the truck to change lane?

You answered correctly. You should let a truck change lane for two main reasons. First, the driver of a truck has limited visibility due to blind spots and may not see your car, making it dangerous if you do not show caution. Second, it's an important part of road safety to show consideration for truck drivers because their vehicles are large and less maneuverable. There is no indication in this question or driving scenario that the alternate merging (zipper) rule applies here; that rule is relevant only where two lanes merge into one. By remembering to give trucks space and consideration, especially when they are changing lanes, you help prevent accidents and support smoother traffic flow.