To measure one wavelength, you would measure the distance between? c and a c and f d and b a and b

A and B

Explanation: The wavelength is defined as the distance between two consecutive equivalent points in the wave. (two consecutive crests, two consecutive valleys, etc)

This would be in the graph the distance between the points A and B, or between E and F.

Then the right answer between the given ones is the fourth: A and B.

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.