I believe the answer is salt. because it is a compound of two distinct elements that can be broken down without changing the properties of substance. A pure substance is a substance that are made up of only one type of an atom or one type of a molecule. Pure substances may be further broken down into elements and compounds.
Potential energy is not in motion, which means the roller coaster wouldn't be moving. do therefore it is kenetic energy because that means movement energy. hope this helps