Giving reason for you selection, select pairs out of the following four waves in a medium which will give rise to (i) beats, (ii) destructive interference, (iii) stationary waves.
(i) ξ1 = Acos 2π(v1t + \(\frac{x}{\lambda_1}\) )
(ii) ξ2 = Acos 2π(v1t + \(\frac{x}{\lambda_1}+\pi\) )
(iii) ξ3 = Acos 2π(v1t + \(\frac{x}{\lambda_2}\) )
(iv) ξ4 = Acos 2π(v1t − \(\frac{x}{\lambda_2}\) )