Answer: Option A
Thorium radioactive pollutant has recently drawn attention of public due to its occurrence in the building material.
Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium.
The alpha particles can travel only very short distances through most materials and cannot go through human skin.