Thomas is the managing director of Tanks Ltd, a company manufacturing trains. Nathan enters into a contract with Tanks Ltd to purchase a locomotive. After having made the contract, it becomes apparent that Thomas's appointment was defective, and that his appointment as managing director is invalid. The contract made by Thomas, on behalf of Tanks Ltd, is...

a. valid, only if the contract was signed by two directors, or one director and the company secretary.

b. valid, because Nathan can assume that Thomas was validly appointed to the position.

c. valid, only if the company ratifies the contract.

d. invalid.