How is the gravitational force similar to the electric force?

They are both proportional to the product of their masses and inversely proportional to the square distance between the masses.

d) physical means can be used to separate the components of air.

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.