To determine blood alcohol concentration for a person, what is necessary to know?

Answer: C — How much the person weighs

To accurately determine blood alcohol concentration (BAC), knowing the person's weight is crucial. This is because a person's body weight significantly influences how alcohol is distributed throughout the body. Generally, individuals with higher body weight can dilute alcohol more effectively, resulting in a lower BAC compared to someone of lighter weight who consumes the same amount of alcohol. While personal habits like favorite drinks, water consumption, and exercise routines might influence alcohol metabolism or hydration levels, they do not provide a direct measurement or understanding of BAC. The critical factor remains weight, as it directly affects the volume of distribution of alcohol in the bloodstream, making it an essential component in calculating BAC accurately.