A common emitter transistor amplifier is connected with a load resistance of \(6 \mathrm{k} \Omega\). When a small a.c. signal of 15 \(\mathrm{mV}\) is added to the base emitter voltage, the alternating base current is \(20 \mu \mathrm{A}\) and the alternating collector current is \(1.8 \mathrm{~mA}\). What is the voltage gain of the amplifier ?
(A) 90
(B) 640
(C) 900
(D) 720