\begin{table}%t3 
 \caption{\label{tab:optical_para}Decay indexes of the optical light curves.}
\par
 %\centering
\par
  \begin{tabular}{llll}
     \hline \hline\noalign{\smallskip}
      GRB        & $T$ (s) &   $\alpha_{\rm O}$\tablefootmark{\dag} & $\chi^2$ / d.o.f.  \\
       \hline
      GRB~071112C& 250 $< T <$ 10~000   & $-$0.95($\pm$0.02)  & 31/44 \\ 
      GRB~080506 & 200 $< T <$ 1300    & $-$0.19($\pm$0.06)  & 59/76 \\ 
            & 1300 $< T < 2\times10^5$ & $-$0.83($\pm$0.03)  & \\ 
            \hline
            \end{tabular}
            \tablefoot{The uncertainties show the 90\% confidence levels of the parameters. 
       \tablefoottext{\dag}{Optical decay index.}} 
\end{table}