Air fares are generally lower on Tuesdays and Wednesdays each week.  What is a likely explanation for this occurrence?

A. Supply is relatively variable, and lower demand on these days leads to a lower equilibrium price.
B. Demand is relatively variable, and lower supply leads to a lower equilibrium price.
C. Lower levels of both supply and demand on these days lead to a lower equilibrium price.
D. Supply is relatively fixed, and lower demand on these days leads to a lower equilibrium price.
E. Demand is relatively fixed, and lower supply leads to a lower equilibrium price.

Answer: D