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.

我选A，答案是B，

可是B中的revert to their earlier tipping habits说的很不好。earlier tipping habits指的是什么？既然说的是Habits,也就是在某种情况的限制下他们不假思索所采取的行动。而这某种情况的限制就是每道菜都写thank you。那么之前做的调查显示，写thank you的有一部分菜人们就会多给tips，那我完全可以把“写thank you的有一部分菜人们就会多给tips”理解为是一种habit，这样的话，对B取非就是加强了~~。

所以还是选A.  A能起到是搭桥作用。

A我觉得把客人硬性分类就不对了，A相当于自己又假设了regular patrons ，occasional patrons两者付小费不同了，孰多孰少呢又？

B正确在于，排除了revert to their earlier tipping habits这个可能，使 seeing “Thank you” written on their bills ，能达到 average income from tips would be significantly higher than it otherwise would have been这个目的。

D 又画蛇添足的把restaurant自己假设分个expensive or not类，没有意义。

题干是先做了个实验，实验的结果是bill上有thank you的那些账单tip多，结论是，如果账单上都加上thank you，那么tip会比没有thank you的时候多。

可是B能算做一个假设吗？如果B不对，那实验的结果是怎么出来的呢？实验就已经说明了加了thank you后客人给得多，也就是不像以前那么给tip呀。没有B结论也成立呀。