\textbf{203.} Which statement about the periodic table of elements is correct?
\begin{enumerate}
\item Liquid elements (under standard conditions) have non-metallic properties.
\item Some elements of the sixth period of the periodic table have not yet been discovered.
\item Two elements $_{22}$Z and $_{22}$X are intermediate elements and belong to the same group.
\item Lanthanides and actinides are placed in the sixth and seventh periods of the periodic table, respectively.
\end{enumerate}