Concept:Air-column instruments produce sound when air inside a pipe or tube vibrates.
Explanation:In a flute, blowing air across the mouthpiece sets the air column inside the pipe vibrating.
In a trumpet, the player’s lips vibrate, causing the air inside its tube or pipe to vibrate.
Piano, guitar, and violin mainly use vibrating strings to produce sound, not air in pipes.
Thus, the pair that works on vibration of air in pipes is the flute and the trumpet.
Answer:D. flute and the trumpet