A glass tube is closed at one end and has a diaphragm covering the other end. The tube is filled with gas and some very fine sawdust has been scattered along inside the tube. When the diaphragm is driven at a frequency of 1457 Hz, the sawdust forms small piles 20 cm apart.

(a) What is the speed of the sound in the gas? 

(b) Do the piles of sawdust represent displacement nodes or antinodes in the sound wave? Explain.