The statement below is followed by four options. Each option is related to the item in the statement, but only one is always true. Select the option that always holds and makes the statement true.
"A sound always has ______."
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A vibrating source
The answer must hold for every sound, without exception. An option that is present in most cases but can be missing in even one case is not always true.
Option (1) A vibrating source — sound is produced by a vibrating object; when the vibration stops, the sound stops, so this is always true.
Option (2) Air to travel through — sound also travels through water and solids; a swimmer hears sounds under water.
Option (3) A loudness that humans can hear — a sound can be too faint for us to hear, and ultrasound, above about 20,000 Hz, is still sound though humans cannot hear it.
Option (4) A musical note — noise, such as the sound of traffic, is sound but not music.