I believe the answer is background radiatin
There are so many routes by which water can move through the various stages in the water cycle.
<h3>What is the water cycle?</h3>
The water cycle has to do with the route of movement of water in the enviroment. We have to note that there are so many routes by which water can move through the various stages in the water cycle.
The sun supplies the energy that drives the water cycle. This is seen in the fact that the supply of energy from the sun is required for evaporation to take place.
Learn more about water cycle:brainly.com/question/1151425
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Cs -137 has a half life of about 30 years. If 60 years pass, there is two half lives passed so 1/2 * 1/2= 1/4. Take 1/4 and multiply it with the mass given:
(1/4)*20mg=5mg left