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D is corrent because once the third party learns the identity of an undisclosed principal, the third party may elect to hold either the agent or the principal liable on the contract.
A is incorrect because an agent acting on behalf of an undisclosed principal is a party to the contract. Rick may, therefore, enforce the contract against Sam.
B is incorrect because specific performance is not an appropriate remedy when money damages will adequately compensate the plaintiff. Plain white paper is not unique; consequently, specific performance is not available.
C is incorrect because Sam did not know that Rick was acting as an agent at the time the contract was entered into; Rick is liable as a party to the contract. The mere disclosure of the existence and identity of the principal after the parties have entered into the contract does not relieve the agent of his contractual obligations.