Given what you know about the transmission of cholera, which water companies' customers would you expect to have cholera at a lower rate?

A. the companies that took their water from a point in the river above where sewage was introduced
B. the companies that were the farthest down the river the companies that took their water from the point
C. in the river that had the smallest amount of traffic by boats
D. the companies that supplied water to both house owners and tenants