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Lanthanoid contraction is mainly caused by

  1. Aincrease in the number of shells across the series
  2. Bpoor shielding of the nuclear charge by electrons
  3. Ccomplete shielding of the nucleus by electrons
  4. Ddecrease in nuclear charge across the series

Answer: B

Across the lanthanoids the nuclear charge rises by one unit per element while the added electrons shield each other poorly because of the diffuse shape of orbitals. The effective nuclear charge therefore rises and the size shrinks steadily from La to Lu.
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