Signals and Systems - Engineering

Q1:

A casual LTI system is described by the difference equation
2y[n] = ay[n - 2] - 2x[n] + βx[n -1]
The system is stable only if

A |a| = 2 |β| < 2

B |a| > 2 |β| > 2

C |a| < 2 any value of β

D |β| < 2, any value of a

ANS:C - |a| < 2 any value of β

How please explain?