Frequent use of cell phones/a cell phone & each of

Wow... I thought I came to a wrong site.

Anyhow, I have two questions to ask.

Q1. Which one is correct or more appropriate? My gut tells me that both are correct but I want to be sure.

A) Frequent use of cell phones can cause brain damage.
B) Frequent use of a cell phone can cause brain damage.

Is it OK to use "brain damages" instead of "brain damage"?

Q2. Are these gramtically correct? If yes, which one is most preferred?
A) Each of the trainees gave presentations on their understanding of the project.
B) Each of the trainees gave presentations on one's understanding of the project.
C) Each of the trainees gave presentations on his/her understanding of the project.

Thanks always...