单项选择题

What is the most popular food in western countries If you are not sure, statistics might give you the answer.
Every year, some 20 million people walk into restaurants and buy 2 billion sandwiches. Americans alone eat more than 1 billion sandwiches annually. If you lined up all the sandwiches Westerners eat in a year, they would go around the world 13 times!
When John Montagu made the very first sandwich in England in 1762, he couldn’t have imagined that it would still be very popular almost 250 years later. There is a campaign starting this month in the US to find the healthiest and best tasting sandwich in the country.
Sandwiches are commonly carried to school or work in lunchboxes or brown paper bags to be eaten for lunch. They are also taken on picnics and hiking trips. Their longlasting popularity lies in their convenience and fast preparation time.
You can put everything you want inside a sandwich. They are easy to clean-up after, nutritious and come in hundreds of different kinds.

The long-lasting popularity of sandwich lies in（）

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