31. Denise is a freelance writer who works with a number of clients. She used to charge by the hour, but she has nowswitched to charging by the page. In other words, she has moved from a _______ schedule.
(A)fixed interval to a fixed ratio
(B)fixed interval to a variable ratio
(C)variable interval to a fixed ratio
(D)variable intervat to a variable ratio

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統計： A(1), B(0), C(0), D(0), E(0) #2788074