latex中beamer里加有框的文字

documentclass[pdf]{beamer}
\usetheme{Copenhagen}
\begin{document}
\begin{frame}
\frametitle{Frame title}
\begin{block}{Observation 1}
Simmons Hall is composed of metal and concrete.
\end{block}
\begin{exampleblock}{Observation 2}
Simmons Dormitory is composed of brick.
\end{exampleblock}
\begin{alertblock}{Conclusion}
Simmons Hall $\not=$ Simmons Dormitory.
\end{alertblock}
\end{frame}
\end{document}

 注意：

\usetheme如果默认为default，则会没有框

上述实现的结果