Final answer:
The aircraft's actual cruising altitude remains constant when flying along the 500-mb surface.
Step-by-step explanation:
The aircraft's actual cruising altitude remains constant when flying from Tucson to Caribou along the 500-mb surface. The 500-mb surface is a level in the atmosphere where the atmospheric pressure is half of the surface pressure. This level is also known as the mid-tropospheric level and is generally located around 5500-6000 meters above sea level. Therefore, regardless of the distance traveled, the aircraft will maintain a constant altitude along this level.