听力原文： Nowadays, at each check-out counter in almost all supermarkets, a new machine is helping customers in a hurry. An electronic system "reads" a bar code on each tin, box or bottle. These little lines of the bar code give the computer all the information it needs -- the kind of product, the price and the tax. The girl at the counter simply moves the groceries along in front of the "eye" of the machine. And the bill is ready in no time.
Most people are very pleased with this new idea. They don't make as many mistakes as people. Some shoppers, how ever, are complaining. They can’t read the price of the food on a tin when all they can see is just little black and white lines. The prices are on the shelves, but they want them on each tin. Many people are not sure that machines are honest shopkeepers! The machines are very expensive. But everyone, even these worried shoppers, will be happy, if in the end, they bring food prices down.
The main idea of this passage is that ______.

A. supermarkets are becoming more and more popular
B. computers find use in supermarkets
C. a new way of checking out in supermarkets
D. people are not used to the new machine