Why input impedance of a MOSFET is very high?

Please show work

Draw the DC circuit:  b)DC computations: Compute the DC collector current of each transistor. I_C 1 = ________________________ mA I_C 2= ________________________ Calculations:  c)Draw the AC circuit and compute gm, r_pi for the system. g_m 1= _______________________ mA/V r_pi 1 = _______________________ kohms g_m 2= _______________________ mA/V r_pi 2 = _______________________ kohms AC circuit:  Calculations: d)Compute the gain A_v=Vo/Vi at the operational band (for sufficiently high frequency) Gain = A_v = ______________________________ Computations: