Mr. A signs the instrument in the following manner. State the instrument which cannot be considered as promissory note. O a. I promise to pay B or order RO 500. O b. I promise to pay B RO 500 seven days after my marriage with C. O c. I acknowledge myself to be indebted to B for RO 1,000 to be paid on demand for value received. d. I promise to pay B RO 10,000 after three months