What is a lone wolf?

[Robert...]

English Idioms and Expressions, Elementary Level

ESL/EFL Test #9 "Animal Idioms", question 10

He doesn't really go around with many people because he's a bit of a lone wolf.

(a) single person
(b) lonely person
(c) unhappy person
(d) simple person

English Idioms and Expressions, Elementary Level

ESL/EFL Test #9 "Animal Idioms", answer 10

He doesn't really go around with many people because he's a bit of a lonely person.

Correct answer: (b) lonely person

Your answer was: correct
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I would disagree with this answer. just because a person enjoys being by themselves does not make them lonely. I would say that this person is a single person not a lonely person.

Robert