What is the velocity of the bob of a simple pendulum at its mean position, if it is able to rise to vertical height of $18 \mathrm{~cm}$ (Take $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^{2}$ )
(A) $0.4 \mathrm{~m} / \mathrm{s}$
(B) $4 \mathrm{~m} / \mathrm{s}$
(C) $1.8 \mathrm{~m} / \mathrm{s}$
(D) $0.6 \mathrm{~m} / \mathrm{s}$