Final answer:
The primary source of radon gas is 2)soil and rock as it is a product of the natural decay of uranium-238 found in these terrestrial materials. It seeps into buildings from the ground and is not primarily emitted by power plants, factories, or vehicles.
Step-by-step explanation:
The primary source of radon gas is soil and rock. Radon-222 is a radioactive gas that is part of the decay series of uranium-238, commonly found in trace amounts in soil and rock.
As it decays, radon gas seeps out of the ground and can accumulate in buildings, particularly in lower levels like basements, posing health risks.
Different from potential sources like power plants, factories, or automobile exhausts, radon's terrestrial origin means that it enters homes through natural means, such as through cracks in foundations or other openings.
It does not predominantly come from industrial or man-made sources like nuclear energy or consumer products. Though it is a component of natural radiation.
Elevated levels indoors can lead to significant health concerns, including lung cancer, thus various mitigation techniques are employed to reduce indoor radon levels.