Smart host question

[avon1982]

HI everyone
This question is from MS press book (pg 10-19 Q2)
You are the administrator of contoso isp You have two smtp server configured as smart hosts:smart1.contoso.com and smart2.contoso.com Your isp manages dns records for Northwind traders whose Exchange 2003 server has FQDN mailserv.nwtraders.com Northwind Traders has permanent connection to your isp HOW should you configure MX records
for this setup in dns
a MX 10 smart1.contoso.com
MX 10 smart2.contoso.com
MX 20 mailserv.nwtraders.com

b MX 10 mailserv.contoso.com
MX 20 smart1.contoso.com
MX 20 smart2.nwtraders.com

c MX 10 smart1.contoso.com
MX 20 smart2.contoso.com
MX 30 mailserver.nwtraders.com

d MX 10 mailserv.contoso.com
MX 10 smart1.contoso.com
MX 10 smart2.nwtraders.com

According to me and as also mentioned in the MS PRESS book on pg 10-8
the answer for MX record in an ISP's Namespace for a permanent connection should be

MX 10 mailserv.nwtraders.com
MX 20 smart1.contoso.com
MX 20 smart2.contoso.com
This option is not mentioned moreover the correct answer is mentioned
as ( b) which does not look right from any angle as DNS names are
mismatched What do you feel ?