A wave pulse travels from medium X into medium Y and then immediately into medium Z. The wave speeds are vX=4m⋅s−1, vY=2m⋅s−1, and vZ=6m⋅s−1. The two boundaries are only 1m apart, so the pulse reflects from each interface before any part exits medium Y. Which description best represents the pulses that return to medium X?
a) Two pulses return; the first is upright and the second is inverted.
b) Two pulses return; the first is inverted and the second is upright.
c) Only one pulse returns; it is inverted.
d) Two pulses return; both are inverted.