A contract for office cleaning services requires the contractor to vacuum all floors nightly. The agency's contract manager verbally tells the contractor's supervisor to also start shampooing the carpets weekly, a task not mentioned in the original scope of work. The contractor performs this extra work for a month and then submits an invoice for it. This situation is best described as a:
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A
Bilateral amendment
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B
Unilateral change order
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C
Constructive change
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D
Cardinal change