Wind or gravitational pull. A wave is made by friction between wind and the water’s surface. Waves can also be made by the sun and moon’s gravitational pull on the earth.
Answer:

Explanation:
Given that,
Frequency of a radio antenna is 1 MHz
Power, P = 21 kW
We need to find the the waves intensity 25 km from the antenna
. The object emits intenisty evenly in all direction. It can be given by :

So, the wave intensity 25 km from the antenna is
.
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Answer:
The correct option is : Their atoms have eight electrons in their valence shells, so noble gases are very unreactive.
Explanation:
The octet rule state that atoms tend to complete their last energy levels with eight electrons, and that this configuration make them very stable and unreactive.
Noble gases are characterized as unreactive atoms, and this is associated with the fact that they have a complete valence shell, it means that they have eight electrons on it (they follow the octet rule).
Atoms with less electrons on their valence shells tend to react with another atom, forming bonds, to complete their valence shells (with eight electrons).