\textbf{80-} \textbf{Scientists do not yet know if there is life of any kind somewhere outside the solar system. This has long been a/an ---------- to us and will most probably continue to be so for many years to come.}\\
1) exploration \hspace{1cm} 2) expectation \hspace{1cm} 3) mystery \hspace{1cm} 4) contact
\noindent