Answer:
The answer is 0.83 seconds.
Explanation:
The formula of free fall is following:

Where g=9.8 m/s^2 and t=2 seconds, the rock takes:

19.6 meters. This is the half distance of the cliff. The whole distance is 39.2 meters. So it takes:

2.83 second to fall down completely. The rock takes the second half of the cliff in 0.83 seconds
What gas? Gas is an invisible form so I don't see how it would have a "defined structure".
Researchers found the "cosmic microwave background radiation", which is a heat imprint left over from the big bang.
The redshift of light emitted by most galaxies indicates the universe is expanding.