American scientist Benjamin Franklin invented the glass harmonica, also known as the Franklin harmonica, in the early 1760s. It is a mechanical version of the much simpler musical glasses that were popular at that time. These were a set of glass bowls of different sizes that were arranged from smallest to largest to produce distinct pitches. The bowls were fine-tuned by filling them partially with water. In the Franklin model, shallow glass basins are attached to a horizontal spindle that is revolved by a crank attached to a pedal. The spindle is placed in a trough of water so that the glasses are kept wet. The shimmering, bell-like sound is produced by touching the fingers to the wet edges. This instrument was popular in the late 18th and early 19th centuries. What is the meaning of the word pitches as it is used in this passage?
A.
baseball tosses
B.
frequencies of sound waves reaching the ear (musical)
C.
the distance between adjacent threads on a screw