Which of the following terms describes the distance between the crests of successive waves?

Distance between two successive crests are called wavelength

wavelength is the distance covered by the particle in one oscillation.

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when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.