Under which of the following conditions will the speed of sound in air be maximum?

Consider the following statements regarding waves: 1. If the particles of a medium move along the direction of propagation of the wave, the wave is called a longitudinal wave. 2. If the particles of a medium vibrate perpendicular to the direction of propagation of the wave, the wave is called a transverse wave. 3. Transverse waves involve changes in the shape/shear of the medium, whereas longitudinal waves involve changes in the density/volume of the medium. Which of the statements given above are correct?

At what speed should a source of sound move away from a stationary observer so that the observer perceives the apparent frequency to be half of the original frequency? (where v is the speed of sound)