Out of the given four waves (1), (2), (3) and (4)y=asin(kx+ωt)       ......(1)y=asin(ωt−kx)       ......(2)y=acos(kx+ωt)      ......(3)y=acos(ωt−kx)      ......(4)emitted  by four different sources S1, S2, S3 and S4 respectively, interference phenomena would be observed in space under appropriate conditions when