请说明数字签名的主要流程。数字签名通过如下的流程进行：
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What do people do when they come to a fast food restaurant？
A. They order and eat their food at the restaurant.
B. They buy and take their food out and eat it in their cars or in their homes.
C. They order, pay for and get their food in their cars.
D. They do one of the three above-mentioned things.
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