请帮纠结的我

sdcar2010

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?




A. 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.



B. Regularly seeing “Thank you” written on their bills would not lead restaurant patrons to revert to their earlier tipping habits.



C. The written “Thank you” reminds restaurant patrons that tips constitute a significant part of the income of many food servers.



D. The rate at which people tip food servers in Pennsylvania does not vary with how expensive a restaurant is.



E. 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    那c为什么不行呢，两者区别是什么

This question is asking for a necessary assumption.  You can use negation method to find the right answer.

If you negate answer B, the original argument falls apart. However, if you negate C, there is no impact on the original argument, meaning C is not necessary for the argument to hold.  Therefore, B is the correct answer while C is not.

sdcar2010

You can try the negation method on the other two problems and see what you will get out of it.