\textbf{168-} The force-time graph of a moving object is as shown below. The average net force exerted on the body during $50$ seconds of given data, how many Newtons is it?
\begin{center}
\textit{[Figure: F(N) vs t(s) graph; F rises from 0 to 40 N between t=0 and t=10, stays at 40 N from t=10 to t=30, then decreases linearly to 0 at t=50]}
\end{center}
\begin{center}
(1) $15$ \qquad (2) $17.5$ \qquad (3) $14$ \qquad (4) $17.5$
\end{center}