Final answer:
When a periodic wave travels from one medium to another, the variables that are likely to change are velocity and wavelength.
Step-by-step explanation:
When a periodic wave travels from one medium to another, the pair of variables that are likely to change are:
Velocity: The velocity of the wave can change as it moves from one medium to another. Different mediums have different properties that can affect the speed at which the wave travels.
Wavelength: The wavelength of the wave can also change when it travels from one medium to another. The properties of the new medium can cause the wave to stretch or compress, resulting in a change in its wavelength.
So the correct answer is A. velocity and wavelength.