\textbf{171-} A gas inside a container at constant pressure $2\times10^5\ \text{Pa}$ is cooled and its volume goes from $6\ \text{lit}$ to $2\ \text{lit}$. If in this process $2800\ \text{J}$ of heat is released, by how many joules does the internal energy of the gas decrease?
\begin{center}
(1) $1200$ \hspace{2cm} (2) $1800$ \hspace{2cm} (3) $2000$ \hspace{2cm} (4) $2600$
\end{center}