Discuss and explain whether there is valid consideration for each of the following promises:
a. A and B entered into a contract for the purchase and sale of goods. A subsequently promised to pay a higher price for the goods when B refused to deliver at the contract price.
b. A promised in writing to pay a debt, which was due from B to C, on C's agreement to extend the time of payment for one year.
c. A orally promised to pay $150 to her son, B, solely in consideration of past services rendered to A by B, for which there had been no agreement or request to pay.