New York City has been experiencing a housing emergency for a long time. Apartments are difficult to come by. In fact, the vacancy rate has been below 5 percent since World War II. Most recently (2018), the rental vacancy rate was at 3.63 percent that is much lower than the national vacancy rate, which was last recorded at an average of 6.9 percent. The vacancy rate is as low as 2.71 percent in some boroughs such as the Bronx. The most likely cause of the housing emergency is: A) a price floor on rent higher than equilibrium price. B) a price ceiling on rent lower than equilibrium price. C) too high incomes in New York City. D) a lack of a rationing mechanism to distribute existing apartments.