His grandparents used to a walk after dinner, but now they are used to a walk after brea

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A. take, take
B. taking, taking
C. taking, take
(答案→)D. take, taking
解析：考察固定搭配，used to do sth 过去常常做某事  be used to doing sth   习惯于做某事，句意：他的祖父母过去常常晚饭后去散步，但是现在他们习惯早饭后散步。故选D