Which of the following is NOT a common oxygen scavenger?

Calcium carbonate is not considered a common oxygen scavenger because its primary function in water treatment is as a pH adjuster and scaling inhibitor rather than an agent that actively removes oxygen. Oxygen scavengers are typically used in boilers and other systems to prevent corrosion by reducing the amount of dissolved oxygen in the water.

Sodium sulphite and hydrazine are both well-known oxygen scavengers; sodium sulphite reacts with oxygen to form sodium sulfate, effectively reducing oxygen levels. Hydrazine is effective in high-temperature applications and is used particularly in boiler systems to scavenge oxygen. Hydroquinone is also recognized for its properties as an oxygen scavenger in very specific applications. Thus, calcium carbonate does not fit into the category of oxygen scavengers, making it the correct response to the question.