Must be wealthy VERSUS must be millionaire

[Kirill]

Business Idiom in English, Intermediate level

ESL/EFL Test #141 "Expressions and phrases with money", question 6

Alan's parents must be .......... They've just bought him a new car.

(a) penniless
(b) wealthy
(c) millionaire
(d) bankrupt

Business Idiom in English, Intermediate level

ESL/EFL Test #141 "Expressions and phrases with money", answer 6

Alan's parents must be wealthy. They've just bought him a new car.

Correct answer: (b) wealthy

Your answer was: incorrect
Alan's parents must be millionaire. They've just bought him a new car.
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I don't understand why can't I use millionaire. It's because millionaire is singular?

Kirill

[Conchita]

That's right, English Learner, a noun would have to be written in the plural form:

Alan's parents must be millionaires.