What does lanthanoid contraction result in?

Larger number of oxidation states are exhibited by the actinoids than those by the lanthanoids,the main reason being

The number of $f$-electrons in the $+3$ oxidation state of gadolinium $(Z=64)$ is $x$ and in the $+2$ oxidation state of ytterbium $(Z=70)$ is $y$. The sum of $x$ and $y$ is:

Fill in the blanks:
$1.$ The atomic radii of the elements of the third transition series are .......... to those of the corresponding elements of the second transition series.
$2.$ Samarium melts at .......... $K$.
$3.$ The color of lanthanoid ions is due to the presence of .......... electrons.

Which of the following elements are included in the actinoid series?

Which of the following statements are correct?
$I$. $P$ has the least negative electron gain enthalpy among $P, S, Cl$, and $F$.
$II$. In $Eu$ and $Yb$, lanthanoid contraction is not observed.
$III$. $Ce(OH)_3$ is the most basic among lanthanoid hydroxides.
$IV$. The radii of $Na$ and $Na^+$ are $95 \ pm$ and $186 \ pm$ respectively.