One the most popular musical instruments is the piano. In the traditional musical instrument classification system of string, woodwind, percussion and brass, the piano falls into two categories. It is considered a string instrument because the sound comes from the strings inside the piano. It is also considered a percussion instrument,since the player strikes the keyboard, which signals an internal hammer to strike the strings. Choose the main idea of the paragraph. a The strings of a piano makes the sound. b The piano is a popular instrument. c The piano is both a string and percussion instrument d The piano is a string instrument.