Which of the Following Gratuitous Promises Is/are Enforceable by Statute

Which of the following gratuitous promises is/are enforceable by statute?

A) A good-faith contract modification in a contract for the sale of goods
B) A written offer signed by a merchant to buy or sell goods that assures it will be kept open for one month
C) A renunciation of a claim in a written waiver that is signed and delivered by the aggrieved party when the ^{contract involves a sale of goods}
D) All of these are enforceable by statute.