We are taught that business letters should be written in a formal style in a personal one.

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(答案→)A. rather than
B. other than
C. better than
D. less than
解析：本题考查副词短语辨析。rather than而不是。根据句意及in a formal style与in a personal style成分一致的线索，可断定本句是想表达“应该……，而不应该……”，故只有D项正确。句意：商务书信必须用正式文体而不是个人文体来写。