Where a party accepts part payment of a debt they may, under the rule in Pinnel's Case (1602), at a later date request payment of the amount outstanding unless the other party provided extra consideration when making the part payment. Which of the following are valid extra consideration for part payment of a debt?

a) An intention by the debtor to be legally bound by the part payment.

b) A guarantee by the debtor to make the payment on the date contractually agreed.

c) Payment in the form of goods rather than cash.

d) Payment by a party other than the debtor.