Gravity causes water to infiltrate until it reaches impermeable zones where it is diverted laterally. Gravity generates the flow of springs, rivers, and wells. If the pores in rocks and sediments are connected, gravity allows the water to move slowly through them.
I think its B. But it is a hard one, I have to admit. Hey if this is a website, just make a teaxher account and get all the answers!
Answer:
I don't know for sure so I am sorry if its wrong.
Explanation:
The preposition is "on". The prepositional phrase is "had spent many hours working on". And it modifies the word "their"
Again I think I am wrong so I am so sorry.