When I worked as a waitress in Chicago, US, my coworkers&nbs

When I worked as a waitress in Chicago, US, my coworkers  and I would groan (叹息) whenever we heard someone with a foreign accent coming into our restaurant. We knew what it meant to serve a non–American: no tip. We would work just as hard as we always did, but we might not get paid
Americans have an unspoken rule about tipping: they give tips to almost everyone who offers them a service of some kind. Americans tip their waiters, barbers and taxi drivers. An appropriate (合适的) tip is between 15 and 20 percent of the amount charged for the service, but the charge for the tip doesn’t appear on the bill. The customer is expected to add it on himself/herself.
So if you order $100 worth of food, you are expected to pay your waiter at least $15, making your total payment $115. Tipping less than this sends a message to your waiter that you think they’ve done a bad job serving you. And to leave no tip at all is simply unacceptable.
It’s not that American waiters are greedy. In many countries, waiters are paid a salary for their work. They’ll earn money even if no one comes into the restaurant. This system offers much more safety for waiters than the American version. In the US, waiters know that a night without customers means a night without pay.
Some countries include a tip for waiters – a “service charge” – on the bill itself. Since the tip is included with the other charges, waiters don’t need to worry about people forgetting to tip. But in the US, waiters do not receive a salary, and service charges only appear on bills when there are six or more customers at the table.
Since almost all American customers are familiar with the system, they know to add a tip without being told. But visitors to the US may expect waiters to be paid a salary, or think that the tip is included on the bill. So as much as we waiters loved hearing stories about other places from our foreign customers, we were always nervous when they got ready to leave the restaurant. We were never sure what to expect.
55. Why would the author and her coworkers groan when they had to serve a non-American?
A. They would have difficulty understanding the customer.
B. They had to work harder to get tips from the customer.
C. They might not get any payment from the customer.
D. They thought a non-American customer would be hard to please.
56. If you order $550 worth of food, you are supposed to pay your waiter at least ______.
A. $55    B. $82.5     C. $100     D.$110
57. Why would it be thought unacceptable not to tip a waiter in the US？
A. They enjoy getting tips from their customers.
B. They work harder than waiters in other countries.
C. They are greedier than waiters in other countries.
D. They earn most of their money from diners’ tips.