The following figure shows the time-acceleration graph for a particle in rectilinear motion. What will be the average acceleration in first twenty seconds?

Question

$\begin{array}{1 1} (a)\; 10 ms^2 \\ (b)\; 15 ms^2 \\ (c)\; 20 ms^2 \\ (d)\; 25 ms^2 \end{array}$