Concept:An idiom is a phrase with a figurative meaning different from its literal words. "To go for a song" means something is sold at a very low price.Explanation:The idiom "to go for a song" describes an item being sold for far less than its true worth. It implies a bargain or a cheap sale. For example, if a rare painting is auctioned for a tiny amount, it "went for a song". The phrase originates from the idea that something is as cheap as buying a simple song. In the given options, "be sold cheaply" exactly matches this meaning. The other options are incorrect: "be expensive" is the opposite, "be unsold" means not bought at all, and "be sung loudly" refers to singing, not selling.Answer:Option C: Be sold cheaply