Concept:Well water is often hard because it contains dissolved calcium or magnesium salts, which prevent soap from lathering well.Explanation:Boiling removes temporary hardness by decomposing soluble calcium hydrogen carbonate into insoluble calcium carbonate, which settles out.This makes the water suitable for laundry because soap can then lather properly.Filtering, chlorination, and adding alum do not mainly remove hardness.Answer:B. boiling.