The density of the water causes pressure, which causes water to move.
There are no options to choose from and this makes it hard to answer the question. I hope my answer is correct and comes to your help. The reason that the new recruit fainted was that he did not use muscular compression to return the blood to the heart and thus his brain became devoid of oxygenated blood, resulting in the new recruit getting fainted.
I'm not sure if this is correct, but the answer I got was: endoplasmic reticulum.
I hope this helps, and have a good night! :D