Following are expressions for four plane simple harmonic waves:
$(i) \, y_1 = A \cos 2\pi \left( n_1 t + \frac{x}{\lambda_1} \right)$
$(ii) \, y_2 = A \cos 2\pi \left( n_1 t + \frac{x}{\lambda_1} + \frac{1}{2} \right)$
$(iii) \, y_3 = A \cos 2\pi \left( n_2 t + \frac{x}{\lambda_2} \right)$
$(iv) \, y_4 = A \cos 2\pi \left( n_2 t - \frac{x}{\lambda_2} \right)$
The pairs of waves which will produce destructive interference and stationary waves respectively in a medium are:

• A
  $(iii, iv), (i, ii)$
• B
  $(i, iii), (ii, iv)$
• C
  $(i, iv), (ii, iii)$
• D
  $(i, ii), (iii, iv)$