**Core Concept**
Sewer gas is a mixture of various gases that are produced by the decomposition of organic matter in sewers, including hydrogen sulfide (H2S), carbon dioxide (CO2), and methane (CH4). The composition of sewer gas can vary depending on factors such as sewage composition, temperature, and microbial activity.

**Why the Correct Answer is Right**
Sewer gas primarily consists of H2S, a toxic gas responsible for the characteristic "rotten egg" odor, CO2, a byproduct of microbial respiration, and CH4, a potent greenhouse gas produced by methanogenic archaea. These gases are produced as a result of the anaerobic decomposition of organic matter in sewers. SO2, on the other hand, is a sulfur dioxide gas that is commonly associated with industrial emissions and air pollution, rather than sewer gas.

**Why Each Wrong Option is Incorrect**
**Option A:** H2S is indeed a component of sewer gas, responsible for its characteristic odor.
**Option B:** CO2 is also a component of sewer gas, produced through microbial respiration.
**Option D:** Methane (CH4) is another major component of sewer gas, produced by methanogenic archaea.

**Clinical Pearl / High-Yield Fact**
It's worth noting that long-term exposure to sewer gas can cause respiratory problems, neurological symptoms, and even cancer. Sewer gas can also be a health hazard for workers and individuals living near sewage treatment plants.

**✓ Correct Answer: C. SO2. Sulfur dioxide is not a primary component of sewer gas.**