Chapter 15, review question 15.1: Why is "xinetd" a correct answer but not "iptables"?

Quiz question 15.1 asks us to select items which can regulate "access to services and ports". The correct answer, according to the quiz-checking program, is "netfilter", "tcpwrappers", and... "xinetd"? I searched the entire text of chapter 15 and found zero references to xinetd, and just one passing reference to inetd. In addition, the item "iptables" is flagged as a wrong answer, but how does iptables fail to qualify as regulating access to services and ports?

My guess is that the correct answer list should include iptables and exclude xinetd. If that's the case, however, the question is still kind of ambiguous. The chapter doesn't cover xinetd, but some students might know about it already, and xinetd configuration can affect service/port availability. If you want one option to be unrelated, it seems better to pick a word that is more clearly unrelated.