ABC Siding,Inc

ABC Siding,Inc.manufactures aluminum siding.ABC enters into a contract to deliver siding to Slippery Siding,Ltd. ,a retailer of aluminum siding.The written agreement insists that all modifications to the agreement be in writing and signed by both parties.This prohibition against oral modifications is

A) valid regardless of whether the clause is signed separately.
B) invalid unless the clause is signed separately by Slippery Siding,Ltd.
C) invalid unless the clause is signed separately by ABC Siding,Inc.
D) invalid unless new consideration is supplied by the parties when the modification agreement is made.