False misrepresentation in a contract

1. A and B entered into a contract with C. A’s consent was obtained only because B had deceived or defrauded him. May A ask for annulment of the contract with C? Why? 2. A made a promissory note in B’s favor. A, however, alleged that the cause was his gambling losses in a prohibited game Who has the burden of proving that the game was indeed a prohibited one? Why? 3. B took the ring of S unlawfully and sold it to X. X knew the voidable title of B, despite of that he bought it. Can X claim error or mistake if S will claim an annulment of a sale? Why? 4. A buyer of land before buying was followed to investigate for himself the boundaries and the true nature of the land, even after the seller had told him about it. The seller did nothing to prevent the investigation from being complete. Later, the buyer brought an action to annul the contract, alleging that false representations had been made to him. Is there a false misrepresentation?