since 一般指既然的意思,常用在句首.如：Since I have stayed in Shanghai for years, I am very familiar with the city.
As 可以引导定语从句,表原因,也一般放句首.如：As he was only 5 years old, he was too young to go to school.
because 一般指原因,放在句子中间,the fuel must have been finished because the engine stopped, because 指原因时,一般不用逗号和前面的句子隔开.此句用for 最恰当.