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Since 1961 the standard reference chosen for measuring atomic masses has been:
ASodium 23
BCarbon 12
COxygen 16
DHelium 04
Answer & Solution
Correct answer: B. Carbon 12
1. Chemists needed one universal reference atom.
2. One isotope was agreed upon in 1961.
3. That isotope is carbon 12.
_Source: syllabus Science Class IX, Chapter 3, Atoms and Molecules._
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