Following are expressions for four plane simple harmonic waves: (i) $y_1 = A\cos 2\pi(n_1t + x/\lambda_1)$ (ii) $y_2 = A\cos 2\pi(n_1t + x/\lambda_1 + \pi)$ (iii) $y_3 = A\cos 2\pi(n_2t + x/\lambda_2)$ (iv) $y_4 = A\cos 2\pi(n_2t - x/\lambda_2)$. The pairs of waves which will produce destructive interference and stationary waves respectively in a medium, are