Answer:
Water is the most important element in our body and we need a constant supply of it to survive. Now in the scenario that you described is very important to make sure you boil the water because that is the best way to ensure that you killed any living organism (Virus, bacteria or parasites) that could endanger your life. So the answer to your question is A as it was said before but let me elaborate a little more.  
Explanation:
Many times, water is contaminated by mosquito or bacteria that can cause among other problems Dysentery. This is an inflammation of the intestines that cause a lot of pain and a bloody diarrhea. This bacteria is spread through water or food and in many third world countries is the main cause of death among the population. When you boil the water the bacteria is killed and the water is safe for drinking. If you ever learn of someone developing such symptoms and they claim to have drank unsafe water, suggest them to go to the doctor intermediately. Their life is at risk. 
 
        
             
        
        
        
<span>c) volcanoes that form on the oceans floor</span>
        
             
        
        
        
A. He should add shells that are different colors, but the same size and type, to the tank and observe which shells the hermit crabs choose.
Durning an experiment you need to keep everything the same but only change the certain things based on your theory. 
Such as how the student wanted to see if the color attracted the crab. If he had changed the size and type of the shell then the crab would have chosen the bigger one as they tend to grow into larger shells.
        
             
        
        
        
Answer:
it's curve1000 for this question