(c) where the defendant entered into the contract under circumstances which though not rendering the contract voidable, makes it inequitable to enforce specific performance. Explanation 1.- Mere inadequacy of consideration, or the mere fact that the contract is onerous to the defendant or improvident in its nature, shall not be deemed to constitute an unfair advantage within the meaning of clause (a) or hardship within the meaning of clause (b). Explanation 2.- The question whether the performance of a contract would involve hardship on the defendant within the meaning of clause (b) shall, except in cases where the hardship has resulted from any act of the plaintiff subsequent to the contract, be determined with reference to the circumstances existing at the time of the contract.