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Number theory Difficulty 1.0 Junior Find the answer Canada

Which of the following is a prime number?

(A) $20$ (B) $21$ (C) $23$ (D) $25$ (E) $27$

Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project. Solutions are the publisher's, linked not copied.