What is a filter?

• A. A porous device for removing impurities from a liquid or gas passed through it
• B. A device used to measure heart rate
• C. A container for storing dry goods
• D. A tool for cutting metal

Answer: (A)

A filter is a porous device that allows a liquid or gas to pass through while trapping impurities. The key idea is selective passage: the pores let the desired fluid move through but block unwanted substances, so the overall result is cleaner liquid or air. This is why filters are used to purify water, clean air, or protect systems from contaminants in health and safety settings. It’s about removing unwanted materials from the flow to reduce exposure or damage. The other items describe completely different functions—measuring heart rate, storing dry goods, or cutting metal—so they don’t fit the purpose of a filter.