A man is running towards a plane mirror with some velocity. If the relative velocity of his image with respect to him is $4 \mathrm{~m} / \mathrm{s}$, then the velocity of a man is :
(a) $2 \mathrm{~m} / \mathrm{s}$
(b) $4 \mathrm{~m} / \mathrm{s}$
(c) $8 \mathrm{~m} / \mathrm{s}$
(d) $16 \mathrm{~m} / \mathrm{s}$