Please select an answer which doesn’t indicate the difference between firm offer and non-firm offer?

A firm offer should be made when sellers promise to sell goods at a stated price within a stated time where non-firm offer not.
B. A firm offer should stipulate an expiry date but non-firm offer’s price is changeable anytime.
C. A firm offer is subject to obligation within validity while a non-firm offer is a kind of quotation without obligation.
D. A firm offer will be accepted but a non-firm offer will not be accepted by the buyer.