Explanation: “stationary” just means something that is not moving or doesn’t move. Power plants down really move however as the others are vehicles they move so they aren’t stationary. So the answer is power plant as it’s the only one that doesn’t move.
The key to understanding this question, is first knowing what "stationary" means. When something is stationary, it doesn't move, it just stays in one place.
Since power plants are buildings, they naturally don't move. On the other hand, the other choices are all vehicles that are intended to move. So while all of these options do cause air pollution, the power plant is the only one that is stationary.
