If a rent ceiling is set at $900 a month, what is the housing shortage? if a rent ceiling is set at $1,500 a month, what is the housing shortage?

You are asked to find the housing shortage if a rent ceiling is set at $900 and $1,500 a month. Since we are not given any other values, I will set an equilibrium rate for the rent ceiling at $1000. You have to know that you cannot answer this problem if you do not know the conditions. The first condition is that if the rent ceiling is above the set equilibrium then it will have no effect in the housing shortage because it exceeded the quantity demanded for the housing. Therefore $1500 has no market shortage. But if the rent ceiling is below the set equilibrium then it creates a housing shortage so $900 has a housing shortage. 

The answer should be b.