Answer:
A. The author uses the first sentence to inform readers of the central idea that people travel by airplanes more now than in the past. 
Step-by-step explanation:
- Airplane travel has increased significantly in the past decades. 
 
- Unlike in the past when airplane travel was very costly, most Americans consider airplane travel a viable option when they are planning a trip.
 
- How is the author's purpose conveyed in the passage?
 - The author uses the first sentence to inform readers of the central idea that people travel by airplanes more now than in the past.