If you blow across the top of an empty soft-drink bottle, a pulse of sound travels down through the air in the bottle. at the moment the pulse reaches the bottom of the bottle, what is the correct description of the displace- ment of elements of air from their equilibrium positions and the pressure of the air at this point? (a) the displacement and pressure are both at a maximum. (b) the displacement and pressure are both at a minimum. (c) the displace- ment is zero, and the pressure is a maximum. (d) the displacement is zero, and the pressure is a minimum.