a. The velocity must remain constant to keep the ratio of frequency and wavelength in check. The wave speed is determined by the properties of the medium through which the wave travels, and is independent of the frequency and wavelength of the wave.
c. 0.35 m. The wavelength can be calculated using the formula: wavelength = wave speed / frequency. Plugging in the given values, we get wavelength = 4.5 m/s / 25 Hz = 0.18 m/Hz = 0.35 m (rounded to two decimal places).