what to do after received multiple NS record?

[MontyRee]

Hello, all.
 
 
If client executes recursive query, client will receive some NS records like below. 
 
 
$ dig www.example.com
 
 
example.com.            172800  IN      NS      a.iana-servers.net.
example.com.            172800  IN      NS      b.iana-servers.net.

if 
 
a.iana-servers.net. is local network and fast
b.iana-servers.net. is remote network and very slow, 
 
 
1. then, client querys most to a.iana-servers.net or randomly?
 
 
2. if client received NS records(a.iana-servers.net and b.iana-servers.net) 
   from root servers, how to check which DNS is fast or slow?
   I can't find any icmp packets or related packets.
 
3. below is a cache_dump.db.
   Anyone can explain what's the meaning of TTL and srtt?
  
 
; J.ROOT-SERVERS.NET [v4 TTL 86393] [v4 success] [v6 unexpected]
;       192.58.128.30 [srtt 18] [flags 00000000] [ttl 1793]
; k.gtld-servers.net [v4 TTL 3] [v4 success] [v6 unexpected]
;       192.52.178.30 [srtt 9] [flags 00000000] [ttl 1793]

 
 
If any RFC or related documents, please let me know.
 
 
 
Thanks in advance.
 
 
 
 
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