Red Seal Plumbing Practice Exam

"Parts per million" refers to the amount of a particular substance in a specific volume of water. Specifically, it means the number of particles of the substance present in one million units of water. Therefore, the options of micrograms, millilitres, and grams are not correct as they are either units of weight or volume, and do not accurately represent the concentration of a substance in a given volume of water. The option of milligrams is also incorrect as it is a unit of weight and does not accurately represent the ratio of particles to water. Therefore, the correct answer is C, as milligrams is the accurate unit to express the concentration of a substance in water for the given scenario.