—Mom，dad is smoking again!—Well，let him cut it gradually since old habits die hard.A. off

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A. off
(答案→)B. down
C. up
D. out
解析：考查短语辨析。cut off  中断cut down 减少，削减，cut up 切碎，cut out 删掉，句意：妈妈，爸爸又吸烟了。好吧。让他逐渐减少吸烟吧因为旧习惯很难除去。故选B。