Deep freezers — why R-12 was preferred over R-22 (legacy systems)? Select the most appropriate reason set for preferring R-12 over R-22 in deep-freeze applications (historical context).

Introduction / Context:
Refrigerant selection for deep freezers balances thermodynamic performance, compressor mechanics, oil return, and safety. Historically, certain CFCs offered an advantageous combination that made them popular in low-temperature domestic and commercial units.

Given Data / Assumptions:

• Legacy (pre phase-out) comparison of R-12 and R-22.
• Deep-freeze conditions (very low evaporator temperatures).
• Hermetic/small compressors typical of domestic or light commercial freezers.

Concept / Approach:
R-12 operated at lower discharge pressures than R-22 for comparable evaporator/condensing conditions, reducing compressor stress. It also provided good oil miscibility across a broad temperature range, important for reliable oil return. Together these factors often produced higher COP in small systems at deep-freeze conditions compared with R-22.

Step-by-Step Solution:
Check operating pressures: R-12 < R-22 at similar conditions → less mechanical load.Check oil return: R-12’s miscibility improves lubrication reliability.Net performance: Under many legacy designs, R-12 yielded higher COP.Hence, all listed advantages are valid.

Verification / Alternative check:
Historical compressor application charts and service manuals reflect lower pressures and good oil behavior with R-12 in low-temp applications.

Why Other Options Are Wrong:
Single reasons (a), (b), or (c) are incomplete; the preference arose from the combination.

Common Pitfalls:
Applying current environmental rules retroactively to historical selection; the question focuses on engineering reasons, not regulatory status.

Final Answer:
all of the above