I think "female" or "male" are countable nouns, however,...
I found a sentence in a textbook ; They are all female. Is it not correct, "They are all females."?

Question

I think "female" or "male" are countable nouns, however,...
I found a sentence in a textbook ; They are all female. Is it not correct,  "They are all females."?

Answer

In this sentence, 'they' is the noun and 'female' is the adjective - so this sentence is correct.

But if 'female' is used as a noun, it is a countable noun :)

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In this case "female" is being used as an adjective, so it is correct! You are right though in that "female" is also a countable noun.

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Sarah is right but in practice they are interchangeable.

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adjective

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I think it should be plural or u can use the word women instead of females