The feedback system with characteristic equation s4 + 20

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The feedback system with characteristic equation
s⁴ + 20 Ks³ + 5s² + 10s + 15 = 0 is

1. stable for all value of K
2. stable for positive value of K
3. stable for ∞ > K > 7.0
4. unstable for all values of K

Correct Option: D

By Routh criterion

For this, any value at K the system is unstable.