All of the following has a naturally occurring decay product in a gaseous form, EXCEPT:

Radon is a naturally occurring decay product of Uranium or Thorium. It is a colorless, odorless, chemically noble gas that can penetrate the eah's crust and accumulate in buildings. Radium is a product of decomposition of Uranium or Thorium, which fuher decomposes into Radon. Know this - Radon decays to products that emit heavy ionizing alpha paicles, which may damage the DNA in respiratory epithelial cells. Radon exposure increases the risk of developing lung cancer, usually the small cell type. Ref: Gibbons D., Pisters K.M., Johnson F., Eapen G.A. (2011). Chapter 15. Non-Small Cell Lung Cancer. In H.M. Kantarjian, R.A. Wolff, C.A. Koller (Eds), The MD Anderson Manual of Medical Oncology, 2e.