How are race and ethnicity are determined in a society ?

Cindy is 18 years old. cindy's parents and her 23-year old brother rich are going away for the weekend. renee, cindy's best friend, is pressuring her to throw a huge saturday night party for all their friends since no one will be home. renee even asked rich if he could pick up a few kegs for them. rich brought back two kegs of beer, a bottle of vodka, and a few bottles of wine, which he hid in the basement. renee also spread the word around school for everyone to show up. cindy now has over $100 worth of alcohol hid in the basement, an empty house for the weekend, and several friends expected to show. what should she do?