certified expert says oxygen is a common noun, all other three answer says oxygen is a proper noun Why - English - Nouns

Question

certified expert says oxygen is a common noun, all other three
answer says oxygen is a proper noun Why is the difference in answer
and what is the correct answer?

Saima Siddiqui · Accepted Answer

Oxygen is a common noun, a general word for any oxygen anywhere

A proper noun is the name of a specific person, place, thing, or a
title; for example:

Oxygen Drive
Roberts Oxygen Company
Oxygen Magazine (women's fitness)
Oxygen Media (the Oxygen Channel)
"Oxygen", a novel by Andrew Miller

certified expert says oxygen is a common noun, all other three answer says oxygen is a proper noun. Why is the difference in answer and what is the correct answer?