引用产生问号？

Question

我正在努力正确显示引文，但到目前为止没有任何效果。输出是?。下面是我的代码。

\documentclass{article}
\usepackage{hyperref}

\begin{document}

\title{Thesis}
\author{Author Name}

\maketitle

\section{Introduction}

This document refer to \cite{einstein} 

\bibliography{TEST}

\end{document}

出去：

这是我的.bib

@article{einstein,
    author =       "Albert Einstein",
    title =        "{Zur Elektrodynamik bewegter K{\"o}rper}. ({German})
    [{On} the electrodynamics of moving bodies]",
    journal =      "Annalen der Physik",
    volume =       "322",
    number =       "10",
    pages =        "891--921",
    year =         "1905",
    DOI =          "http://dx.doi.org/10.1002/andp.19053221004",
}