which Name sever is selected?

[houguanghua]

If there is a list of NS records, the local name server uses the RTT (round trip time) algorithm to find the fatest, and queries that server. 
But I found it's not right. In the testing, the local name server doesn't query the fastest authority name server. Some one tells me that if the local name server gets the RTT to one remote server is les than 30ms, it will not test RTT to other remote servers, even if the RTT is more less. In other words, the local server will only query the first remote server with the RTT less than 30ms. Who would tell me the truth? Thanks! Guanghua 		 	   		  
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