In the figure shown AB is a rod of length 30 cm and area of cross-section 1.0 cm2 and thermal conductivity 336 S.I. units. The ends A and B are maintained at temperatures 20°C and 40°C respectively. A point C of this rod is connected to a box D, containing ice at 0°C, through a highly conducting wire of negligible heat capacity. The rate of heat flow through the wire (in watt) is :