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题库 / GMATAdvanced-CR-31
Clearbell Telephone provides slow-dialing (SD) service to customers for a low fee, and fast-dialing (FD) service to other customers who pay a somewhat higher fee. FD technology, however, is so efficient that it costs Clearbell substantially less per average call to provide than does SD. Nonetheless, accountants have calculated that Clearbell’s profits would drop if it provided FD to all its customers at the current low-fee rate.
Assume that installation costs for FD are insignificant if the customer already has SD service. Which of the following, if true about Clearbell, best explains the results of the accountants’ calculation?
The extra revenue collected from customers who pay the high fee is higher than the extra cost of providing SD to customers who pay the low fee.
The low fee was increased by 6 percent last year, whereas the higher fee was not increased last year.
Although 96 percent of customers regard FD service as reliable and more convenient than SD, fewer than 10 percent of them choose to pay the higher fee for FD service.
The company’s competitors generally provide business customers with FD service at low-fee rates.
Profits rose slightly each month for the first three months after FD was first offered to customers, then fell slightly each month for the succeeding three months.
文章：C电话公司有一个费用低的SD服务，和一个费用高的FD服务。
FD技术提高了，所以每个FD电话的成本比SD低了。
答案是，会计计算得出，C的收益会降低，如果所有客户都用FD服务，但收取低费用。
问在安装费不变的情况下，怎么解释？
猜想：Profit↑=revenue-cost↓
SD revenue低，FD revenue高
现在cost（FD）P (SD)，但是为什么如果都让客户用FD，profit反而会下降？注意这里影响both SD and FD，因为换成FD的技术之后，SD的收费还是不变的，但是FD收入下降了。
A：从付高费用的顾客得到的额外revenue比额外cost（SD）高。这里有一个很隐晦的点——
Profit（SD）=r（不变）-C↓
Profit（FD）=R（变成low fee）-C↓
所以这里无法判断cost下降了多少。也就无法判断profit增多或者减少。A说[R（FD）-R（变成low fee）]>[cost(SD)-cost↓]，这里明确了变化范围，所以ΔR > Δcost, 所以收入减少了。
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