I can't believe what he says because he is changing his mind. .A. regularlyB. steadilyC.

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A. regularly
B. steadily
C. suddenly
(答案→)D. constantly
解析：    
试题分析:考查副词辨析。A定期地；B稳定地；C突然地；D持续地。根据语境判断D选项正确，因为他不断地改主意才不能相信他，句意：我不能相信他说的话，因为他总是改变主意。