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Which two actinoid isotopes are long-lived enough to occur in rocks?

AThorium-232 and uranium-238
BMendelevium and nobelium
CFermium and einsteinium
DLawrencium and curium
Answer & Solution
Correct answer: A. Thorium-232 and uranium-238
Th-232 (1.4 x 10^10 years) and U-238 (4.5 x 10^9 years) have half-lives long enough to persist on earth.
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