As shown in the figure, the circuit is in a steady state. When t=0, the switch S is suddenly turned on (cut off the circuit). The initial voltage on the inductor is uL(0+)=________V. Choose A,B,C or D. Before the swith S is opened, the circuit has been in a steady state, the inductor can be treated as short wire. So the direct current in the circuit is 3/1=3A. After the switch S is opened, at t=0+, the inductor can be treated as a current source with 1 A current. So the voltage on the inductor can be calculated from the 2 Ω resistor , the 3 V voltage source and the 1 Ω resistor in the anti-clockwise direction.