Out of the given four waves (1), (2), (3) and (4)

$\mathrm{y} = \mathrm{a} \sin (\mathrm{kx}+\mathrm{\omega t}) \left(1\right)$

$\mathrm{y} = \mathrm{a} \sin (\mathrm{\omega t}-\mathrm{kx})$                     (2)

$\mathrm{y} = \mathrm{acos}(\mathrm{kx}+\mathrm{\omega t}) \left(3\right)$

$\mathrm{y} = \mathrm{a} \cos (\mathrm{\omega t}-\mathrm{kx})$                   (4)

emitted by four different sources ${\mathrm{S}}_1, {\mathrm{S}}_2, {\mathrm{S}}_3 \mathrm{and} {\mathrm{S}}_4$respectively, interference phenomena would be observed in space under appropriate conditions when