Two masses $m_{1}$ and $m_{2}$ are attached to a string which passes over a frictionless smooth pulley. When $m_{1}=10 \mathrm{~kg}, m_{2}=6 \mathrm{~kg}$, the acceleration of masses is $\quad$
(a) $20 \mathrm{~m} / \mathrm{s}^{2}$
(b) $5 \mathrm{~m} / \mathrm{s}^{2}$
(c) $2.5 \mathrm{~m} / \mathrm{s}^{2}$
(d) $10 \mathrm{~m} / \mathrm{s}^{2}$