How can convince my parents to let me buy GTA 4?

I really want GTA 4 and I've benn trying to get it for about 2 years now. I'm 15 and I played it at a friends house when I was 13 and it was so much fun and really want it. My dad doesn't about me getting it and he said it looked quite good but my mum on the other hand is completely against it and I need some ideas on how to persuade my mum to let me buy it. I'm good at school and get A and B grade for exams and I know that they trust me because they sometimes leave me alone at home for the night and trust me not to have parties and wreck the place. I've got MW2 and Black Ops and they've watched Quentin Tarantino films with me and other 18 films but my mum is totally against GTA 4 and I don't know why but any ideas on how to get the game are appreciated.