The correct answer is peer pressure, although the price of the phone and advertising can also influence your decision. Peer pressure is usually the most important factor for high school students like you, since adolescents usually follow the trend and want to be cool. The price of the phone is also influential but it depends whether you can afford any phone or not, that is it might be a variable to consider or not. Advertising is certainly influential, though it depends on whether you pay attention to them or not.