The answer is motion, this is what I would go with because when you are dealing with gases it puts motion in the term of particles.
Iodine 131 is a radioisotope with a very short half-life of 8.02 days, making it highly radioactive. Frequently used in small doses in thyroid cancers therapies, it is also one of the most feared fission products when accidentally released into the environment. Radiotoxicity of iodine 131.
B. Sugar is a pure substance.
It is sugar because sugar is made up of one atom or molecule.
A) penny is not correct because a penny is made up of copper, zinc, and tin.
C) ocean water is not correct because its made out of water (h2o) and salt.
D) iced tea is not correct because it takes many atoms to combine in order to make tea, then you have to add water and ice to make it an iced tea.
Hope this helps! :)