\textbf{159-} The position--time graphs of two particles A and B are shown below. At what moment is the speed of particle B equal to the speed of particle A? (The graph of particle B is drawn to a scale of one unit.)
\textit{[Figure: position-time graph with two curves A and B, time axis marked at $t=4$ and $t=12$]}
\begin{itemize}
\item[(1)] 10
\item[(2)] 8
\item[(3)] 6
\item[(4)] 5
\end{itemize}