The correct option for above statement is:
Gravity and Inertia
Explanation:
For instance, Gravity from our sun holds us at an essentially same range all the way around. It makes sure we don't sling off into the wisdom of our solar system. Inertia, keeps us in movement, if there were no inertia, our revolution would stop at a standstill. If we stayed, we would be sucked into the sun by its gravity.
Increase, decrease, or remain the same
1 whole wave is 1 crest and 1 trough.
20 crests and 20 troughs is 20 whole waves.
20 waves / 0.4 seconds = 50 waves per second.
Frequency = 50 Hz