XC=∞ , open circuit At low frequency 3CC is open circuited and effect the low frequency response of amplifier At high frequency, short circuit and work as high pass filter. At low frequency CE is open circuit then reduce gain and at high frequency, short circuited so no reduction in gain and work as high pass filter. So effect at low frequency as resonance. In RC coupled common emitter amplifier coupling capacitance and by pass capacitance effect the low frequency response.