For each of the following observations, determine whether supply, demand, or both shifted and how.

a. Over the last decade, the price of hybrid electric vehicles decreased, while the number of hybrid vehicles sold increased. 

b. During winter, the number of daily air routes between Denver and Aspen, Colorado, increases, as does the price of a ticket. 

c. From January through April in the United States, personal certified public accountants (CPAs) see a dramatic increase in their number of billable hours, but the average hourly rate of a CPA remains the same.