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We study linear perturbations about a dust filled Bianchi type-I model with the vorticity set to zero.In comparison to linear perturbations about the Friedmann-Lema(i)tre-Robertson-Walker models,modes of perturbations about the Bianchi type-I models are coupled.We find that the tensor that represents the background shear needs to be degenerate in order for the scalar-mode perturbations to decouple from the rest of the flow.