what to do after received multiple NS record?

[MontyRee]

So thanks for your kind answer.
 
 
>> 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?
>
> only by sending queries and seeing reply.

But if I tested, I can't find any related packets.
after flushing the all cache, I executed like below.
 
$ dig @127.0.0.1 www.example.com   (192.168.1.3 is dns client)

19:23:49.711259 IP 192.168.1.3.12825> 192.54.112.30.domain:  58548 [1au] A? www.example.com. (44)
19:23:49.987665 IP 192.54.112.30.domain> 192.168.1.3.12825:  58548- 0/2/3 (124)
19:23:49.987903 IP 192.168.1.3.29328> 192.112.36.4.domain:  57596% [1au] A? a.iana-servers.net. (47)
19:23:49.987971 IP 192.168.1.3.29630> 192.112.36.4.domain:  57429% [1au] A? b.iana-servers.net. (47)
19:23:50.139827 IP 192.112.36.4.domain> 192.168.1.3.29328:  57596- 0/13/16 (532)
19:23:50.140108 IP 192.112.36.4.domain> 192.168.1.3.29630:  57429- 0/13/16 (532)
19:23:50.140265 IP 192.168.1.3.11208> 192.48.79.30.domain:  56734% [1au] A? a.iana-servers.net. (47)
19:23:50.140476 IP 192.168.1.3.64270> 192.48.79.30.domain:  19705% [1au] A? b.iana-servers.net. (47)
19:23:50.207358 IP 192.48.79.30.domain> 192.168.1.3.11208:  56734- 1/5/4 A 192.0.34.43 (212)
19:23:50.207365 IP 192.48.79.30.domain> 192.168.1.3.64270:  19705- 1/5/4 A 193.0.0.236 (214)
19:23:50.207703 IP 192.168.1.3.58817> 193.0.0.236.domain:  59823 [1au] A? www.example.com. (44)
19:23:50.482477 IP 193.0.0.236.domain> 192.168.1.3.58817:  59823*- 1/2/3 A 208.77.188.166 (140)
 
 
as you see, two dns serves are 192.0.34.43 and 193.0.0.236. but dns client only 
querys to 193.0.0.236 without any reply from 192.0.34.43.
 
 
 
Thanks again.
 
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