Ozone that protects against UV radiation is found in the stratosphere. In this layer, UV light drives the formation of ozone from oxygen molecules, creating a relatively concentrated ozone layer roughly 10 to 50 kilometers above the Earth's surface. This ozone absorbs much of the Sun’s ultraviolet radiation, especially UV-B and UV-C, preventing most of it from reaching the surface. The troposphere contains some ozone but is dominated by weather and life-supporting gases, and the ozone there is not the protective shield; it can be harmful at ground level. The mesosphere and exosphere have much less ozone and aren’t where the protective layer resides. So the stratosphere is the layer containing the ozone that shields us from UV radiation.

In a vapor‑compression refrigeration cycle, the evaporator cools the space by absorbing heat, and the refrigerant leaves the evaporator as a low‑pressure, low‑temperature vapor. To keep the cycle moving, that vapor must be compressed to a higher pressure so it can release its heat in the condenser. Thus, the next component after the evaporator is the compressor. The metering device is placed before the evaporator to drop pressure, while the condenser (and any receiver) come after the compressor. So the immediate next path for the refrigerant is into the compressor.

Ternary blends involve three components. In refrigeration, a ternary blend is a mixture of three different refrigerants chosen to achieve specific pressure, temperature glide, and performance characteristics. That’s why describing a blend of three refrigerants is the right characterization. A blend of two would be binary, a blend of four would be quaternary, and a single refrigerant is not a blend at all. An example of a ternary blend is R-407C, which combines three different refrigerants to meet desired cooling performance.

Hydrofluorocarbons are refrigerants made from carbon, hydrogen and fluorine (no chlorine). R-134a fits this group as 1,1,1,2-tetrafluoroethane, a commonly used HFC refrigerant. The other options aren’t HFCs: R-600A is a hydrocarbon, R-1234yf is a hydrofluoroolefin (an HFO, not an HFC), and R-12 is a chlorinated refrigerant (a CFC). This combination of elements and the absence of chlorine is what makes R-134a an HFC.