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