Which of the following causes sounds that you hear when someone plays a cello with bow?

The sounds that you hear when someone plays a cello with a bow are caused by the vibration of the cello's strings. When the bow is drawn across the strings, it causes them to vibrate, and this vibration produces sound. The pitch of the sound is determined by the length and tension of the string, and the timbre of the sound is determined by the shape and size of the cello's body and the type of bow that is used.