In a Kundt's tube demonstration of stationary sound waves, fine powder on the bottom of the tube accumulates in heaps at the nodes. A student measures the distance from the first heap to the ninth heap to be 84.0 cm, as shown in the diagram.
What is the wavelength of the sound waves?
10.5 cm10.5\text{ cm}10.5 cm
18.7 cm18.7\text{ cm}18.7 cm
21.0 cm21.0\text{ cm}21.0 cm
42.0 cm42.0\text{ cm}42.0 cm