Because of gravity... if you want a more detailed answer look it up on google!
C is the answer both a and b
Im not really good with essays,but i can give you an idea.
I think the best way to learn things is to find an interest in it and continue doing some research on it until you have a good understanding of it.Thing like a new sport or experiment
Approximately 97.5% of the earths water is salt water (saline), and 2.5% is fresh water hope this helps.