Which one of the following substances used in dry cleaning is a better strategy to control environmental pollution?

First, let us recall that conventional dry-cleaning of clothes has long been carried out with the solvent tetrachloroethylene, commonly called perchloroethylene or simply “perc.” However, environmental chemists have shown that $$\text{C}_2\text{Cl}_4$$ is a volatile chlorinated hydrocarbon; when it evaporates it adds harmful organic vapours to the atmosphere and its residues in soil and groundwater are toxic to living organisms. Therefore, modern industry looks for substitute fluids that clean garments equally well but release far fewer pollutants.

With this goal in mind, researchers examined several other small molecules.

We have the option $$SO_2$$ (sulphur dioxide). Although $$SO_2$$ is not chlorinated, it is still an acidic oxide; when it dissolves in atmospheric moisture it forms sulphurous and sulphuric acids, leading directly to acid rain. Hence using $$SO_2$$ would aggravate air-quality problems rather than solve them.

Next we look at $$NO_2$$ (nitrogen dioxide). This brown gas is an important component of urban smog; photochemical reactions involving $$NO_2$$ produce tropospheric ozone, another major pollutant. So employing $$NO_2$$ in dry cleaning would likewise worsen air pollution.

The usual industrial solvent, $$C_2Cl_4$$ (tetrachloroethylene), produces chlorinated emissions and can persist in the environment for years, contaminating soil and aquifers. Its vapours are also suspected carcinogens. Therefore, continuing with $$C_2Cl_4$$ does not constitute a “better” environmental strategy.

That leaves $$CO_2$$ (carbon dioxide). In everyday discussion $$CO_2$$ is linked to the greenhouse effect, but here it is used in a closed dry-cleaning machine under high pressure, where it exists as liquid or supercritical $$CO_2$$. After the cleaning cycle the gas can be re-compressed and recycled inside the machine, so there is practically no release to the atmosphere. Because $$CO_2$$ is non-toxic, non-flammable, inexpensive, and already abundant in the air, its controlled reuse in such equipment avoids introducing new harmful chemicals and thus represents the most environmentally benign alternative among the four substances listed.

Therefore, choosing carbon dioxide for dry cleaning is the superior method to control environmental pollution.

Hence, the correct answer is Option B.