Two pipes of inner diameter 100mm and outer diameter 110mm each are joined by flash butt welding using 30V power supply. At the interface, 1mm of material melts from each pipe which has a resistance of 42 Ohms. If the unit melt energy is 64.4 MJ/m3, then time required for welding (in S) is
(a) 1 (b) 5
(c) 10 (d) 20