From Mr. Ward Hoffman.
Sir, I was halfway through Professor Raj Persaud's article “What's the tipping point"
(Financial Times Weekend, April 9-l0) when it occurred to me that what I was reading was not ironic. If Prof Persaud wants to know why Americans tip in restaurants, he need only ask the first American he meets in London.
Americans tip in restaurants for one reason, and one reason only: we tip to supplement the salary of restaurant workers. Quality of service does not enter into it, beyond the fact that one may tip a bit less for poor service, or a little more for good service.
Not tipping at all in a non-fast-food restaurant is not a choice. In the US, one used to tip about 15 per cent for dining in a family-style restaurant or in an upmarket restaurant. Here, in San Francisco Bay area restaurants, we are encouraged to tip 20 per cent or more, to help restaurant workers live in this very expensive area.
After eating at an Italian restaurant in my city, I left a tip of 20 per cent on the non-tax part of our dinner bill. It was expected. There is nothing more complicated than that about Americans tipping in restaurants.

1. What can we learn from Hoffman's letter?
A. Quality of service determines tipping in the US.
B. Americans don't tip in non-fast-food restaurants.
C. Tipping in US upmarket restaurants is unnecessary.
D. How to tip in the United States is not complicated.

Which one is the answer? And please explain the sentence "There is nothing more complicated than that about Americans tipping in restaurants." Thank you!
I think the exercise is for you to find the answer by examining the text. Give us your answer, and we'll check it.

There is nothing more complicated than that = that is the most complicated explanation. Here, the explanation for American tipping is very simple, so the writer is being ironic; his meaning is: the reason is not complicated at all.
Thank you, Mister Micawber.