From the frequency response of an amplifier(CE or CS), we can easily observe that it acts as a band pass filter, with two (dominant) corner frequencies. This means the corresponding transfer function has a zero and a pole.
So, now the existence of zero makes the transfer function zero at that particular frequency. So what actually contributes to this zero and at what frequency the output goes to 0??
its simple just put the transfer function equal to zero and then find when it would go zero if any frequency componet comes in numerator then it would just a simple equation [numerator =0 ] and solve it this would give a fixed value for zero
if this is not the situation then see for a frequency componet in the denominator and see what can you do to make it infinity so that overall function would be 0 but in this case normally zero occurs at infinity