# How to put comma between affiliations number of author

I build a paper with

\documentclass[a4paper,12pt]{article}

\begin{document}
\title{This is a Test Paper}
\newcommand*{\affmark}[1][*]{\textsuperscript{#1}}
\newcommand*{\email}[1]{\texttt{#1}}
\author{
Author A\affmark[1] \affmark[2], Author B\affmark[2], Author C\affmark[2], Author D\affmark[2]\\
\email{\{a, b, c, d\}@tju.edu.cn}
}
\maketitle

\end{document}


It generates

The problem is how to put a comma between 1 and 2 following Author A?

Comma can be inserted within \affmark[1,]...