Electrical conductivity of pure germanium can be increased by 1) increasing the temperature 2) doping with donor impurities 3) radiation with ultra violet rays. 4) doping with acceptor impurities

Question

$(a)\;1\;and\;2 \;only \\ (b)\;2\;and \;3\; only \\ (c)\;all \;1,2,3 and\; 4 \\ (d)\;3\;and\;4\; only$