5. The following are the essential requisites of contracts, except: a. Consent of the contracting parties. b. Object certain which is the subject matter of the contract. c. Cause of the obligation which is established. d. Qualified offer of one of the parties which constitutes a counter-offer. 6. The following are requisites for a cause to be valid, except. a. It must be qualified by one of the parties. b. It must exist at the time of the contract is entered into. c. It must be lawful. d. It must be true or real. 7. The following are special forms of payment, except: a. Dacion solvendi. b. Dation in payment. c. Payment in cession. d. Tender of payment and consignation