During an experiment, a student observes the formation of a precipitate. why does this indicate that a chemical reaction is likely occurring?

The formation of a precipitate is always caused by a chemical reaction between a liquid solution and a precipitant. When solid particles suddenly appear in a sample of liquid under experimentation, that is an obvious sign that a reaction has occurred. Precipitates possess various qualities such as color and size. The only exception to this is when a precipitate forms when solubility of a solute is exceeded.

Compound with sulfer  is bright red 

The correct answer should be "electrons orbit the nucleus in circular motions exactly as shown on the classic bohr model."