Two rods whose lengths are $l_1$ and $l_2$ with heat conductivity co-efficients $k_1$ and $k_2$ are placed end to end. The heat conductivity coefficient of a uniform rod of length $l_1+l_2$ whose conductivity is same as that of the system of these two rods is

Statement $A:$ Convection involves flow of matter within a fluid due to unequal temperatures of its parts.

Statement $B:$ A hot bar placed under a running tap water loses heat due to effect of convection within water.

Statement.$\mathrm{C}:$ Heat transfer always involves temperature difference between two systems.