Wind instruments produce sound by causing a column of air within the instrument to vibrate. The player's breath sets this air column into oscillation, and the length and shape of the air column determ

Wind instruments produce sound by causing a column of air within the instrument to vibrate. The player's breath sets this air column into oscillation, and the length and shape of the air column determine the pitch of the sound produced.

• (A) A piece of wood might be part of the instrument's construction (e.g., a clarinet), but it is not the primary source of sound vibration.
• (B) Stretched strings are used in string instruments (e.g., guitars, violins) to produce sound.
• (C) An air column is the vibrating element in all wind instruments (e.g., flutes, trumpets, clarinets).
• (D) Stretched membranes are used in percussion instruments (e.g., drums) to produce sound.

The final answer is $\boxed{\text{C) air column}}$.