Why Is the Agent Able to Be Held Liable on a Contract

Why is the agent able to be held liable on a contract when the existence of the agency arrangement was not disclosed?

A) so that the outcome in a contract situation is the same as that for a tort
B) to discourage the use of undisclosed agency arrangements
C) to offset the fraud frequently present in this situation
D) because the third party could rely on the reputation of only the agent when entering into the contract
E) to ensure that the principal fulfills its obligations under contracts negotiated by authorized agents