Tourist Visa Q's

Hi there,

I am intending to travel to Shanghai and stay with a friend for 3 months. I have a written invitation letter from her. However, when I went to the Chinese Embassy my application for a 90 day stay was rejected. Why is this the case? Should I be applying for a single entry or a multiple entry visa for 3-6 months? The website says that I am allowed to stay on a tourist L for a maximum of 3 months, but why did the embassy reject my application for a 3 month stay?