Based on demand and supply rules, the first scenario C. accounts for the quantity of coast-to-coast flights increasing at the same time that the price was decreasing, irrespective of the relative size of the shifts of supply and demand.
The second scenario A. can't account for the quantity of coast-to-coast flights increasing at the same time that the price was decreasing because an increase in demand and a decrease in supply will both push the price higher.
The trend is that the number (supply) of flights increased and prices decreased.
Prices decrease when demand for flights decrease. Prices also decrease when supply of flights increase. Both situations will therefore lead to a decrease in price regardless of the quantity they occur in.
If the demand for flights increase, the price of flights will increase as well. A decrease in supply will also lead to prices rising.
Both scenarios will therefore lead to a rise in price and so cannot account for the fall in price.
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