用新的子图索引替换

Question

我有以下代码：

\documentclass[12pt,a4paper,twoside]{book}
\usepackage{graphicx}
\usepackage{subfigure}

\begin{document}

\begin{figure}
\centering  
\subfigure
{
    \includegraphics[width=3cm]{example-image-a}
}
\subfigure
{
    \includegraphics[width=3cm]{example-image-a}
}
\subfigure
{
\includegraphics[width=3cm]{example-image-a}
}
\subfigure
{
\includegraphics[width=3cm]{example-image-a}
}
\subfigure
{
    \includegraphics[width=3cm]{example-image-a}
}
\subfigure
{
\includegraphics[width=3cm]{example-image-a}
}
\subfigure
{
\includegraphics[width=3cm]{example-image-a}
}
\subfigure
{ 
\includegraphics[width=3cm]{example-image-a}
}
\subfigure[]
{
\includegraphics[width=3cm]{example-image-a}    
}
\subfigure[]
{
\includegraphics[width=3cm]{example-image-a}
}
\subfigure[]
{
\includegraphics[width=3cm]{example-image-a}
}
\subfigure[]
{
\includegraphics[width=3cm]{example-image-a}
}
\caption{test}
\end{figure}

\end{document}

生成此代码时，它会打印最后一行的索引，即 (i)、(j)、(k)、(l)。我希望它们更改为 (a)、(b)、(c)、(d)