Two masses m and m `(mgtm)` are connected by massless flexible and inextensible string passed over massless and frictionless pulley. The acceleration of centre of mass is
A. `((m_(1)-m_(2))/(m_(1)+m_(2)))^(2)g`
B. `(m_(1)-m_(2))/(m_(1)+m_(2))g`
C. `(m_(1)+m_(2))/(m_(1)-m_(2))g`
D. Zero