In Mendeleev's to the then, why was there no mention of noble gases like He, Ne and Ar?

The valency of noble gases is

Describe in detail how noble gases are isolated from air?

Why are elements of Group-18 known as noble gases?

Explain why the noble gases cannot be liquefied easily.

Why is the group to which the noble gases belong termed as zero group ?

Why are the majority of known noble gas compounds contain xenon?

Why are the majority of known noble gas compounds contain xenon?

This question has statement I and statement II of the four choices given after the statements choose the one that best describes the two statements Statement -I:noble gases have highest ionization enthalpies in their respective period Statement-II:noble gases have stable closed shell electronic configuration

Mention the process to be adopted for the concentration of sulphide ore, why?