Three different alarm clocks X, Y and Z were put at three different places. Sound (in decibels) they produced, when the alarms rang, is plotted in the given graph. Sound produced by clock Y could not be heard because
AThe clock was put in a beaker containing water
BThe clock was put in a cardboard box
CThe clock was put in a jar with vacuum
DThe clock was put in a plastic bag.
What is vacuum?