"Which", "who", and "what" as interrogatives can be either singular or plural; (examples including, "Which is the highest hill?" "Which are the highest hills?" "Who was born in 1920?" "Who were king and queen in 1920?") however, "who" and "what" often take a singular verb regardless of any supposed number. The questions "Who wants some cake?" and "What's in the bag?" do not presuppose anything about number in possible responses: "I want some cake", or "All of us want some"; and "A rabbit is in the bag", or "Five coins and a bus ticket".