The relationship between cell phone use and the incidence of brain tumors is still unclear, this is because some studies show a causal relationship while others do not.

a) is still unclear, this is because some studies show
b) is still unclear; some studies show
c) are still unclear because some studies show
d) is still unclear because: some studies show
e) is still unclear; the reason is that some studies show

how to decide b/n semicolon vs comma??
wat is independent and dependant clause