Read the Paragraph Below and Answer the Questions That Follow

Read the paragraph below and answer the questions that follow.
Traditional musical instruments can be divided, according to how they produce sounds, into three categories: stringed, wind, and percussion. Stringed instruments produce music through the vibration of taut strings that are plucked, strummed, or bowed. The harp, the guitar, the banjo, and the violin are examples of stringed instruments. The second category contains the wind instruments, which are usually sounded by the player's breath. This category includes the clarinet, the tuba, the trumpet, and so on. The third category, percussion instruments, makes sounds when they are struck. Obvious examples of percussion instruments are drums, gongs, and cymbals. Surprisingly, the piano is also considered a percussion instrument because, when the player touches a key, a small hammer inside the piano strikes a string.
On what basis does the writer classify musical instruments?

A) their size
B) how they produce sounds
C) how difficult they are to play