The silicon-controlled rectifier can be turned off

ANS:B - by forced commutation

SCR can be made to conduct heavily by applying a small +ve potential to the gate. in this condn, junctn j2 is reverse biased and j3 is forwrd biased. with this, the anode current increases as the gate current starts flowing..and in an extremely small time j2 breaks down and scr conducts heavily.. the gate also loses control .. even if the gate voltage is removed, the anode current doesn't decrease at all. so the only method to stop conduction is to reduce the applied voltage to zero.