Q30:

In a study conducted in Pennsylvania, servers in various restaurants wrote “Thank you” on randomly selected bills before presenting the bills to their customers. Tips on these bills were an average of three percentage points higher than tips on bills without the message. Therefore, if servers in Pennsylvania regularly wrote “Thank you” on restaurant bills, their average income from tips would be significantly higher than it otherwise would have been.

Which of the following is an assumption on which the argument relies?

1. The “Thank you” messages would have the same impact on regular patrons of a restaurant as they would on occasional patrons of the same restaurant.
2. Regularly seeing “Thank you” written on their bills would not lead restaurant patrons to revert to their earlier tipping habits.

3. The written “Thank you” reminds restaurant patrons that tips constitute a significant part of the income of many food servers.
4. The rate at which people tip food servers in Pennsylvania does not vary with how expensive a restaurant is.
5. Virtually all patrons of the Pennsylvania restaurants in the study who were given a bill with “Thank you” written on it left a larger tip than they otherwise would have.

B感觉是WEAKEN，去掉NOT是ASSUMPTION吧，求指教

B是对的啊.

题目的前提:有THANK YOU的BILL会增加SERVERS的收入.

B的意思是如果所有的BILL上都有THANK YOU,而顾客不会因此恢复以前的小费标准(就是不再多给小费),那么SERVERS确实会增加收入.

这道题可以看成NOT＋削弱就比较容易了。

按住NOT不看，意思就是顾客还会恢复以前给小费的习惯，也就是少给，削弱了结果，

再加上NOT，排除他因，就是假设了。

那为什么A 不行呢？

Thank you的Impact 对regular patrons和occasional patrons都一样。那就排除了一种差异性。

这也可以是结论的Assumption 啊？