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\begin{table}[!htbp]
  \centering
    \caption{Comparison of the CCSHR estimators based on beta and logit-normal distributions}
    \label{tab:addlabel}
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    \begin{tabular}{@{}l@{\hspace*{0.5cm}}ccc@{\hspace*{0.5cm}}ccc@{}}
        \toprule
      & \multicolumn{3}{c}{pH 1.4} & \multicolumn{3}{c}{pH 2.2} \\
        \cmidrule(lr{.5cm}){2-4}\cmidrule(r{-0.0cm}l{0cm}){5-7}
        & \multicolumn{1}{c}{\textit{N} (nm)} & {\textit{C}\textsuperscript{'}} (M)& \textit{D} & \textit{A} (m) & \textit{D}\textsuperscript{'} (m) & \textit{A} \\
        {Molecule AB} & \multicolumn{1}{c}{2.0 ± 0.2} & {7.9 ± 0.6} & {4.7 ± 0.1} & {7.8 ± 0.3} & {26 ± 6} & {0.87 ± 0.09} \\
        {Molecule C} & \multicolumn{1}{c}{9.2 ± 0.2} & {25.9 ± 1.4} & {2.07 ± 0.04} & {1.8 ± 0.3} & {70 ± 5} & {0.19 ± 0.06} \\
        \bottomrule
    \end{tabular}
\end{table}

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