# numbering a several line “equation”

I'm trying to number an equation, which spans over several lines. I need for it to look like this

My current tex for this is:

\documentclass[a4paper,12pt]{article}

\begin{center}
{\it If A is true, then B is true\\
A is true \\
\line(1,0){140}
\\Therefore, B is true''}
\end{center}

-

\documentclass[a4paper,12pt]{article}
$$\begin{tabular}{c} If A is true, then B is true\\ A is true \\\hline Therefore, B is true'' \end{tabular}$$