\begin{table}%t4
%\centering
\par
\caption{\label{table4}Exponential scale lengths $L$  and radial gradients of dust-to-gas ratios and the abundance [O/H].  }
\begin{tabular}{ l l l l l } 
\hline\hline
\noalign{\smallskip}
Ratio  & $R$ & ~~~~~~~~~~ ~C  &  $L$ & Gradient \\
   & (kpc) &  & (kpc) &(dex/kpc) \\
\hline
$\tau_{{\rm H}\alpha}$/$N$(HI) & 0--16 &  1.95~$\pm$~0.08 & 6.2~$\pm$~0.1 & 0.070~$\pm$~0.001  \\
                                 & 5--15 &  1.97~$\pm$~0.08 & 6.1~$\pm$~0.2 & 0.071~$\pm$~0.001 \\
& &  &\\
$\tau_{{\rm H}\alpha}$/$N$(gas) & 0--16 &  1.35~$\pm$~0.05 &7.3~$\pm$~0.2 &  0.059~$\pm$~0.002 \\
                              & 5--15 &  1.27~$\pm$~0.06 & 7.4~$\pm$~0.2 & 0.059~$\pm$~0.002 \\
 & &  &\\
$[{\rm O/H}]\times 10^{-4}$    & 5--15  & 2.95~$\pm$~0.35 &  9.7~$\pm$~2.6 & 0.045~$\pm$~0.012  \\
%
\hline
\end{tabular}
\tablefoot {Here Ratio($R$)~=~C.~exp$(-R/L)$. {Units of} $N$(HI) and $N$(gas) are  10$^{21}$~at~cm$^{-2}$. Errors are standard deviations.}
\end{table}