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Which of these integers cannot be expressed as the difference of two squares?

(A) $5$ (B) $7$ (C) $8$ (D) $9$ (E) $10$

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Source: UK Mathematics Trust, licensed © UK Mathematics Trust; past papers published free at ukmt.org.uk. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project. Solutions are the publisher's, linked not copied.