Final answer:
The blood alcohol concentration (BAC) limit for someone 21 years old or older is 0.08% in most US states.
Step-by-step explanation:
The blood alcohol concentration (BAC) limit for someone 21 years old or older varies depending on the country or state. In most US states, including California, the legal BAC limit is 0.08%. This means that if a person's BAC is 0.08% or higher, they are considered legally intoxicated and it is illegal for them to drive. It is important to note that alcohol affects individuals differently, so even a BAC below the legal limit can impair a person's ability to drive safely.