Groups and FieldsEasyGroups and the Group Axioms

Groups and the Group Axioms · Easy problem

A group of order $4$ with identity $0$ has the Cayley table below; one entry is marked $X$ .

\begin{array}{c|cccc} * & 0 & 1 & 2 & 3\\\hline 0 & 0 & 1 & 2 & 3\\ 1 & 1 & 0 & 3 & 2\\ 2 & 2 & 3 & 0 & X\\ 3 & 3 & 2 & 1 & 0\end{array}

Every row and column of a Cayley table contains each element exactly once. What is $X$ ?