Which is a true statement about salt water?
a. it is a pure substance.
b. it is chemically combined.
c. it is formed using a set ratio of components.
d. it is made up of more than one pure substance

I think the best answer from the given choices is option D. Salt water is made up of more than one pure substance. It consists of salt and water. When each component exist alone in a container they are considered as pure substance however when mixed they are now a solution.

The answer is bhope this

Heat is involved in chemical change in two ways; chemical changes either result in heat being added to or removed from an object, and chemical changes require a certain amount of heat before they can take place.