Answer:
B. - GLASS
When you take your finger away from the glass, the the glass walls continue to vibrate, thus, still producing a musical note. However, if you stop rubbing the rim and leave your finger touching the glass, your finger will stop the vibrations and the sound stops as well.
The tone is created because the tap deforms the side of the glass where it touches. The vibration travels around the glass and deforms the air inside the glass as it moves, producing a sound wave. The shape of the glass and the thickness of its walls control the tone.
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