We finally managed to make the customers of the quality of the vehicle.A. to convinceB. c

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A. to convince
B. convincing
C. convince
(答案→)D. convinced
解析：convince sb. of sth. 使某人确信某事，句中的customers作make的宾语，后面却宾补，而convince与the customers之间是被动关系，因此选D。