我这里有一个代码：
\documentclass[letterpaper, doublespacing, 12pt]{article}
\usepackage[margin=1in]{geometry} % set page parameters appropriately
\usepackage{subcaption}
\captionsetup{font={bf,small},skip=0.25\baselineskip}
\captionsetup[subfigure]{font={small}, skip=1pt, singlelinecheck=false}
\usepackage{graphicx}
\newcommand\msk{\medskip}
\begin{document}
\begin{figure}[htb!]
\centering
\begin{subfigure}{0.47\linewidth}
\centering
\text{\footnotesize{$c = 30\%$}}\par\medskip
\caption{}
\label{fig:PPa}
\includegraphics[width=\linewidth, height=0.15\textheight, keepaspectratio]{PPSh.png}
\end{subfigure}%
\begin{subfigure}{0.47\linewidth}
\centering
\text{\footnotesize{$c = 80\%$}}\par\medskip
\caption{}
\label{fig:PPb}
\includegraphics[width=\linewidth, height=0.15\textheight, keepaspectratio]{PPSh80.png}
\end{subfigure}
\medskip
\begin{subfigure}{0.47\linewidth}
\centering
\caption{}
\label{fig:PPc}
\includegraphics[width=\linewidth, height=0.15\textheight, keepaspectratio]{PPIa30.png}
\text{\footnotesize{Time (days)}}\par\medskip
\end{subfigure}%
\begin{subfigure}{0.47\linewidth}
\centering
\caption{}
\label{fig:PPd}
\includegraphics[width=\linewidth, height=0.15\textheight, keepaspectratio]{PPIa80.png}
\text{\footnotesize{Time (days)}}\par\medskip
\end{subfigure}
\caption{State trajectories of human personal protection model.}
\label{fig:personalprotectiongraph}
\end{figure}
\end{document}
输出结果如下：
我不想将第一行标记为 (a) 和 (b)，而是希望将左列标记为 (a) 和 (b)，将右列标记为 (c) 和 (d)。有办法解决这个问题吗？