Final answer:
When the government sets a maximum price of $45 in the market, a shortage of 21 units would arise.
Step-by-step explanation:
The question refers to the concept of price controls. When the government sets a maximum price of $45 in the market, it creates a situation where the actual price is below the equilibrium price. In this case, option A is the correct answer: a shortage of 21 units would arise. This shortage occurs because the low price encourages buyers to demand more units than what suppliers are willing to supply at that price.