Consider the following techniques.
1. Bode plot
2. Nyquist plot
3. Nichol's chart
4. Routh-Hurwitz criterion

Which of these techniques are used to determine relative stability of a closed loop linear system?
(a) 1 and 2
(b) 1 and 4
(c) 1, 2 and 3
(d ) 2, 3 and 4