Concept:Chlorine bleaches only in the presence of moisture, because it forms a bleaching agent with water.Explanation:Chlorine gas reacts with water to produce hypochlorous acid:Cl2+H2O⇌HCl+HOClHypochlorous acid (HOCl) is the active bleaching agent, releasing nascent oxygen that oxidises coloured substances.Dry chlorine does not bleach, so water is essential.Answer:B. water