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Due to lanthanide contraction, there is a poor shielding of 4f electrons. In this phenomenon, the inner-shell electrons shield the outer-shell electrons in a way that their nuclear charge is not affected. Due to weaker shielding, the positively charged nucleus has a greater attraction to the electrons which leads to the decreased atomic radius on increasing the atomic number. Hence corresponding elements of second and third transition series have similar atomic radii. The correct option is B and A