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Start with the full category, then apply the exact boundary: Use A, B, AB or O. Rh-positive and Rh-negative are a separate classification; the letter O is not zero. You only need one qualifying answer. A longer name is not automatically rarer.
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Shortest accepted input at 65+ points
ABAB · 80 points · 2 characters
ABO group AB has both A and B antigens.
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AB80 points · Shortest accepted spelling: AB
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- Apply the complete question boundary: Use A, B, AB or O. Rh-positive and Rh-negative are a separate classification; the letter O is not zero.
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Each entry belongs to the bounded set defined by the prompt. The named source supports the underlying facts; our editorial scoring is a separate layer.
Source: NHS · Blood groups ↗
Reference version: 2026-10-21. See our answer policy for how we handle aliases and omissions.