The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence. 

London's underground railway will replace commuters' sweaty odors with the gentle waft of perfume when it introduces a new air freshener into the world's oldest subway. London has imported a powerful perfume, called Madeleine, from the Paris Metro. Madeleine will be used to mask the pungent smells of London's underground.
Perfume masks （） odors.