Here's why:
* "Who" is a subject pronoun. It's used to refer to the person performing an action. For example: "Who ate the last cookie?"
* Direct objects receive the action of the verb. For example: "I ate the cookie."
Instead of "who," use "whom" as the direct object:
* "The teacher gave a prize to whom?"
* "I met whom at the party?"
Remember: "Whom" is the object form of the pronoun "who".