纵差动保护又称为环流法纵差动保护的原因是（   ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69378' and id < 4491503 and id<>'4491503' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69378' and id > 4491503 and id<>'4491503' order by id asc limit 5