How to narrow down the research topic

In the first stage, I will focus or pay attention on young customers’ eating habit toward fast food. The age of young customers that I targeted is from 18 until 30 years old. After choosing young customers as my target, I also will narrow the fast food places at Mac Donald and KFC (Kentucky Fried Chicken) in Kuching. Therefore, I decide to describe the effect on consumed too much fast food to my target audiences. The main reason I explain these effects to young customers because in my understanding and observation, there are a lot of young customers were getting fat and facing some diseases nowadays, especially obesity. I am interesting to know the reasons “why” young customer more prefers fast food than other food.