The increase in the atomic radii of the third $(5d)$ series of transition elements is very small, which may be accounted for by the filling of '$X$' orbitals before '$Y$' orbitals. $X$ and $Y$ are:

  • A
    $4f$ and $5d$
  • B
    $5f$ and $5d$
  • C
    $5d$ and $4f$
  • D
    $4f$ and $4d$

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