A buried surface of erosion with older strata above and younger strata below. A buried fault or fracture with younger strata above and older strata below. A buried surface of erosion separating younger strata above from older strata below. It is not harmful as long as you don't walk barefoot in the basement. It has a daughter nucleus that is also radioactive.
All radon gas has a parent nucleus of lead.
Both b and c above. Describe the factors that influence radon gas concentration in residential housing.