1. 原因请见第一段第一行:
The favorite food in the United States is the hamburger.
2. 原因见第二段第二行和第三行:
second, they are fast. People who are busy do not want to spend time.
3.原因见第一段第五行末尾和第六行开始：
The greatest in number（大多数） sell hamburger, French fries and so on.
4. 原因见第二段第二行和第三行：
People who are busy do not want to spend time... waiting while someone prepares it. 忙人们不愿花时间等候别人准备食物.由此可见,食物是实现准备好的.always总是/永远是,这个词太过绝对,万一不凑巧,还没准备好呢?
5. 第一段主要内容是,美国人喜欢快餐店,美国有许多快餐店,快餐店卖各种各样食物.总之就是快餐店很流行.The idea of a fast food restaurant is so popular that nearly every kind of food can be found in one.
第二段主要内容——美国人喜欢快餐店的原因.Fast food restaurant are popular because they reflect(反映) American life style.
两者合并,所以选A.