This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to put freon in an ac unit
Check for vents blowing warm or room temperature air.
This is a tell-tale sign that your unit needs a refill of refrigerant. However, a broken thermostat may cause the same problem.

Choose your answer from:
[A]. If it's a refrigerant issue, the air temperature will steadily grow warmer over time because your unit is slowly losing refrigerant. If a broken thermostat or other issue is causing the warm air, the temperature of the air will probably spike or fluctuate.;
[B]. Check for vents the next time you have an unscheduled problem or have to shut the unit down. Shut down the ac valve, using the air conditioner faucet or oven mitts to close it.;
[C]. For instance, a broken heating unit may have access to the very top of a cabinet under a firelight. The leak may seem to occur from below you but/or in the electrical system, with the model you're buying still not operating with your thermostat.;
[D]. Most broken thermostats are air conditioners that have a broken vent or mixer. This is because a homemade model will pass the air over the exhaust pipe, rather than down to the floor or inside the unit.;
The answer is:
[A].