Which of the following greenhouse gases has no natural source?

Halocarbons have no natural source, being purely human made compounds.

Carbon dioxide arises naturally from respiration, volcanoes and ocean exchange.

Methane is released naturally by wetlands, termites and ruminant digestion.

Nitrous oxide is emitted naturally by soils and oceans through microbial action.

Halocarbons such as chlorofluorocarbons were manufactured for refrigeration and aerosols.

Besides warming the climate, many halocarbons also destroy stratospheric ozone.