I'm using overleaf and I have to plot 2 square, as done with the code below. I'm not able to do the following:

* put the two squares in the center of the pages. I try with `\begin{center}` and `\begin{figure} \centering` but it doesn't work;
* add the text "A" in the middle of the white section of the first square and add an arrow under the second square with letters B and C at the beginning and at the end of the arrow.

Could anyone help me?

Thanks!

```
\begin{multicols}{3}
\begin{tikzpicture}
%Nodes
\draw (0,0) -- (4,0) -- (4,4) -- (0,4) -- (0,0);
\fill[black] (2,0) rectangle (4,4);
\end{tikzpicture}
\columnbreak
\columnbreak
\begin{tikzpicture}
%Nodes
\draw (0,0) -- (4,0) -- (4,4) -- (0,4) -- (0,0);
\shade[left color=white,right color=black] (0,0) rectangle (4,4);
\end{tikzpicture}
\end{multicols}

```