One of the criteria for a valid assignment of a sales contract to a third party is that the assignment must
a. Be supported by adequate consideration from the assignee.
b. Be in writing and signed by the assignor.
c. Not materially increase the other party’s risk or duty.
d. Not be revocable by the assignor
Answer：C
Assignment is the transfer of a right under a contract by one person to another.
Answer (a) is incorrect because consideration is not needed for valid assignment; this is known as a gratuitous assignment.
Answer (b) is incorrect: Normally an assignment is done in writing, but any act, oral or written, is suffi cient if it gives clear intent of the assignment. Only situations included under the Statute of Frauds are required to be in writing.
Answer (d) is incorrect, although when consideration is given in exchange for an assignment, it is irrevocable. However, as a general rule a gratuitous assignment is revocable unless it is evidenced by a writing signed by the assignor, effected by a delivery of a writing used as evidence of the right (i.e., bill of lading), and the assignment is executed. A contract right cannot be assigned if it would materially change the risk or burden of the obligor.