\textbf{78--} Which of the following statements is correct?
\textbf{A:} An element is a substance made up of identical isotopes.
\textbf{B:} About 78 percent of unidentified elements are found in nature.
\textbf{C:} About 6 percent of lithium found in nature is composed of its lighter isotopes.
\textbf{D:} Atoms whose ratio of proton number to neutron number is greater than $1/5$ are unstable.
\begin{itemize}
\item[(1)] ``A'' and ``D'' \hspace{1cm} (2) ``A'' and ``B'' \hspace{1cm} (3) ``C'' and ``D'' \hspace{1cm} (4) ``B'' and ``C''
\end{itemize}