A 'no oral modification' clause states that the contract may be changed only by a signed writing. Its practical purpose is to:

解析

The clause exists to force changes into writing so that a later dispute is settled by a document rather than by two memories — the same discipline B&P §7159(d) makes mandatory on home improvement work, where an unsigned change order is unenforceable against the buyer. (c) overstates it: Civil Code §1698(b) allows an oral agreement to modify a written contract to the extent the oral agreement is executed by the parties, and waiver or a course of conduct can defeat the clause, so 'impossible to modify' is wrong even on a commercial job. (b) confuses a documentation rule with a pricing mechanism; the price moves only if the change order says so. (a) confuses it with a warranty disclaimer, a wholly separate clause.