Passive Filters - Engineering

Q1:

An RC low-pass filter consists of a 120  resistor and a 0.002 F capacitor. The output is taken across the capacitor. The circuit's critical frequency is

A 333 kHz

B 633 kHz

C 331 kHz

D 60 kHz

ANS:B - 633 kHz

a) In order to find the critical frequency what is the need of including resistance (fc = 1/(2*pi*R*C)).

b) We can easily find the critical frequency as
xc= 1/ ( (2* (22/7) fc.

What is the correct method we need to follow?