How would you account for the following?

(i) The variable oxidation states of transition elements are due to the participation of ns and (n-1)d electrons in bonding. Lower oxidation state is exhibited when ns electrons take part in bonding. (ii) This is because the atomic radii of 4d and 5d transition elements are nearly the same. This similarity in size is a consequence of lanthanide contraction. Because of this lanthanide contraction, the radius of Hf becomes nearly equal to that of Zr. (iii) Transition elements act as good catalysts in chemical reactions because they can lend electrons or withdraw electrons from the reagents, depending on the nature of the reaction. The ability of transition metals to be in a variety of oxidation states, the ability to interchange between these oxidation states, and the ability to form complexes with the reagent and be a good source for electrons make transition metals good catalysts.