Fred entered without knocking and, very out of breath, sank a chair. A. onB. offC. intoD.

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A. on
B. off
(答案→)C. into
D. to
解析：本题考查动词与介词的搭配。“sink into sth”意思是“渗入，陷入，沉入；投入……”等意思，“sink into a chair”意思是“（一屁股）坐到椅子上”。其它搭配均不恰当。