If the equilibrium quantity of t-shirts and the equilibrium price per t-shirt both fall, which is the most likely reason?

The graph below shows the monthly market for t-shirts.

If the equilibrium quantity of t-shirts and the equilibrium price per t-shirt both fall, which is the most likely reason?
Consumers’ incomes increase a t-shirt is a normal good.
The per-unit price of cotton used to produce t-shirts rises.
The per-unit price of polo shirts falls.
The price of a pair of jeans rises.

Answer

•   The price of a pair of jeans rises. 
  Explanation:

• Consumers’ incomes increase a t-shirt is a normal good. – False – A normal good is one whose demand increases as income increases, hence if the commodity above was a normal good, an increase in income would be indicated by rise in equilibrium quantity.
• The per-unit price of cotton used to produce t-shirts rises. – False – This would increase cost of production which would rise the price of t-shirts
• The per-unit price of polo shirts falls. – False – The effects of this cannot be determined on the above product
• The price of a pair of jeans rises. – True – A pair of jeans is considered a complement to t-shirts, which means an increase in the price of pair of jeans, would lower the quantity demanded for jeans and lower demand for t-shirts as well. A decrease in demand shifts the demand curve to the left decreasing both the equilibrium quantity as well as the equilibrium price

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