Final answer:
Ammonia refrigerant drums do not typically exhibit frost, fog, or noise as indicators of being nearly empty; the refrigerant level is usually monitored through pressure and temperature gauges, with significant pressure drops being a key indicator of low refrigerant. So the correct answer is option (C).
Step-by-step explanation:
The student asked about the first sign that the drum of ammonia refrigerant was almost empty. In a refrigeration system, ammonia acts as the refrigerant. Refrigerant drums are equipped with measures to indicate the level of substance remaining. The first sign that an ammonia refrigerant drum is nearly empty would not be frost, fog, or noise, as these symptoms could also indicate various states of operation or malfunctions. Typically, the level of refrigerant would be monitored through gauges that display the pressure and temperature of the ammonia in the drum or by weight. If the pressure inside the drum drops significantly and approaches atmospheric pressure, it may be an indication that the drum is nearing emptiness, as most of the liquid ammonia has been vaporized and used.