对闭锁式方向纵联保护来说，收不到信号是保护动作的(  )条件。（0.5）

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69416' and id < 4494623 and id<>'4494623' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69416' and id > 4494623 and id<>'4494623' order by id asc limit 5