When a trace gas becomes absolutely necessary, which of the available gases should be used to help locate a leak? (A) A) Nitrogen with HCFC R-22 (B) B) R-22 due to its universal system compatibility and its miscibility with various oils (C) Nitrogen with HFC R-410A (D) D) Nitrogen with a trace amount of the system's design refrigerant

Using nitrogen with a trace amount of the system's design refrigerant is key because it allows the technician to closely replicate the conditions within the system without introducing an excess of refrigerant, effectively pinpointing leaks. This method maintains the integrity of the system while using the trace gas to provide an accurate reading of leaks. In practical terms, this technique is especially effective in diverse systems where precise leak detection is crucial. By using a compatible refrigerant mixed with nitrogen, technicians can identify problem areas without causing system contamination, making it a preferred method among industry professionals looking to ensure optimal performance.