We can't avoid negative thoughts，but we should make it a habit to replace each negative thought with a positive ．
A. one
B. ones
C. the one
D. that
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(答案→)A. one
B. ones
C. the one
D. that
解析：A 考查代词的用法。根据句意，空格处应表示可数名词“thought”之意，为避免重复，用one来代替。ones 代指名词的复数形式；the one表示特指，that特指单数名词。