函数f（x）= log2x，x＞0  2x-a，x≤0 有且只有一个零点的充分不必要条件是（  ）

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='19886' and id < 1726782 and id<>'1726782' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='19886' and id > 1726782 and id<>'1726782' order by id asc limit 5