两道prep的逻辑题？？？please help!

thanks!

17.   (26864-!-item-!-188;#058&003368) 选 C.

At present the Hollywood Restaurant has only standard-height tables.  However, many customers come to watch the celebrities who frequent the Hollywood, and they would prefer tall tables with stools because such seating would afford a better view of the celebrities.  Moreover, diners seated on stools typically do not stay as long as diners seated at standard-height tables.  Therefore, if the Hollywood replaced some of its seating with high tables and stools, its profits would increase.

The argument is vulnerable to criticism on the grounds that it gives reason to believe that it is likely that

(A) some celebrities come to the Hollywood to be seen, and so might choose to sit at the tall tables if they were available

(B) the price of meals ordered by celebrities dining at the Hollywood compensates for the longer time, if any, they spend lingering over their meals

(C) a customer of the Hollywood who would choose to sit at a tall table would be an exception to the generalization about lingering

(D) a restaurant's customers who spend less time at their meals typically order less expensive meals than those who remain at their meals longer

(E) with enough tall tables to accommodate all the Hollywood's customers interested in such seating, there would be no view except of other tall tables

Which of the following, if true, casts the most serious doubt on the traveler's argument?

(A) Air Dacentaria's flight from Pederton to Dacenta is nonstop, whereas Southern Airways' flight touches down in Gorinda on the way.

(B) Most Southern Airways flights, but not its flight from Pederton to Dacenta, use Sarmouth airport, which because of its mountainous location is one of the world's most dangerous.

(C) For its flights from Pederton to Dacenta, Southern Airways uses a different model of airplane, with a smaller capacity, than the model Air Dacentaria uses for its flights on the same route.

(D) Only in the last few years has the cost of flying from Pederton to Dacenta been more expensive on Air Dacentaria than on Southern Airways.

(E) Although the frequency of accidents is greater on Southern Airways, on both airlines the proportion of flights that have accidents is very small.

17. C 客人逗留的时间长了，虽然收费高，也不能保证能得到更多的利润

18.B 因为S.A航空公司航班除P->D外的机场坐落在mountainous location, 所以才比较危险，言下之意,即P-〉D是安全的，且S.A的比较便宜，所以削弱

thanks! 还是没明白17.

17 题问题的题意： The argument is vulnerable to criticism on the grounds that it gives reason to believe that it is likely that

这个on the grounds that it gives reason to believe that it is likely that 是 修饰文中的aregument还是 criticism?

C中说客人是exception,就不是会逗留呀？