A block of mass $m$ is moving or. a wedge with the acceleration ast. The wedge is moving with the acceleration $a_{1}$. The observer is situated on wedge. The magnitude of pseudo force on the block is?
(a) $m a_{0}$
(b) $m a_{1}$
(c) $m \sqrt{a_{l}^{2}+a_{1}^{2}}$
(d) $m\left(\frac{a_{1}+a_{0}}{2}\right)$