The lanthanide contraction explains which of the following periodic trends?

(a) The atomic radii of the transition metals first decrease and then increase when moving horizontally across each period.

(b) When forming ions the transition metals lose their valence s orbitals before their valence d orbitals.

(c) The radii of the period 5 transition metals (Y–Cd) are very similar to the radii of the period 6 transition metals (Lu–Hg).