Final answer:
a) High
Major air carriers typically have high annual revenues, despite potential fluctuations due to various factors such as fuel costs and travel demand.
Step-by-step explanation:
The question addresses the characteristic of annual revenues of major air carriers. Major air carriers are typically associated with high annual revenues. Revenues can fluctuate due to a variety of factors, such as fuel costs, global economic conditions, and travel demand.
However, despite these fluctuations, major carriers generally have high revenues due to their extensive operations, large fleets, and broad customer base.
Therefore, the correct answer to the question is that major air carriers are characterized by annual revenues that are high.