正常情况下尿素的肾清除率为（ ）。

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SQL Error: select id,body,options from ***_enewstiku where sourceid='31197' and id < 2506896 and id<>'2506896' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='31197' and id > 2506896 and id<>'2506896' order by id asc limit 5