Velocity time graph of a particle in straight line motion is in shape of a sernicircle of radius $R$ as shown in figure. Its average acceleration from $t=0$ to $t=R$ is
(a) $0 \mathrm{~m} / \mathrm{s}^{2}$
(b) $1 \mathrm{~m} / \mathrm{s}^{2}$
(c) $R \mathrm{~m} / \mathrm{s}^{2}$
(d) $2 R \mathrm{~m} / \mathrm{s}^{3}$