Assertion: The pair of elements Zr-Hf, Nb-Ta, Mo-W, ..., Pd-Pt, Ag-Au possess almost the same values of atomic radii. Reason: The occurrence of lanthanoid contraction cancels the expected increase in the atomic radii values from Hf to Hg.

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If both Assertion and Reason are CORRECT and Reason is the CORRECT explanation of the Assertion. Explanation: The assertion is correct because pairs like Zr-Hf, Nb-Ta, Mo-W, Pd-Pt, Ag-Au have almost identical atomic radii. This is due to lanthanoid contraction, which offsets the expected increase in atomic size from the 4d to 5d series. Hence, the reason correctly explains the assertion.

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