Easy looking puzzles can be hard #shorts

Which of the following compound can cause hardness in water ?

By adding which of the following process, permanent hardness of water can be removed.

Problem-solving can be easy and fun for the students if

A student of taxonomy was puzzled when told by his professor to look for a key to identify a plant. He went to his went to his friend to clarify what 'key' the professor was referring to? What would the friend explain to him?