The. is the time it takes for a wave to complete one cycle

Time period

Explanation:

The time taken by a wave to complete one cycle is called the time period of a wave. It is denoted by T. The number of vibration per second is called the frequency of the wave.

The relation between the time period and the frequency is inverse i.e.

If the number of vibrations increases its time period will decrease.

Similarly, for a simple pendulum the time period to complete one cycle is :

l and g are length of pendulum and acceleration due to gravity.

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answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.