Billy tapped the ruler and it made a funny noise. billy heard the sound because the ruler vibrated molecules back and forth. a) the air b) his ear c) the table d) his finger

Billy heard the sound because the air molecules vibrated back and forth as the sound is a type of vibration which needs a medium to travel and the medium, in this case, is the air.

As the rule is tapped the molecules of the air start vibrating back and forth and thus, the sound is propagated along the medium and, reaches Billy's ear due to which he hears the funny noise made by the ruler.