'member of the gym' vs 'member by the gym'

[Ummy]

Business English Courses, Elementary Level

ESL/EFL Test #42 "Basic Business English", question 5

She is a member ......... the gym.

(a) for
(b) from
(c) by
(d) of

Business English Courses, Elementary Level

ESL/EFL Test #42 "Basic Business English", answer 5

She is a member of the gym.

Correct answer: (d) of

Your answer was: incorrect
She is a member by the gym.
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why of and not by?

Ummy

[Mister Micawber]

.
We are members 'of' organizations, not 'by' organizations, Ummy.
.
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Native English teacher at Mister Micawber's