Provide an explanation at the subatomic level to explain why different colors are observed for different metal salts.

When you heat an atom, some of its electrons are "excited* to higher energy levels. When an electron drops from one level to a lower energy level, it emits a quantum of energy. ... The different mix of energy differences for each atom produces different colours. Each metal gives a characteristic flame emission spectrum.

Explanation:

the answer is:  

line graphs show patterns in data with variables that change over time; whereas, circle graphs show data as parts out of a whole (such as out of 100%).