大全5-19，为何选A？望NN指教

19.   Recent estimates predict that between 1982 and 1995 the greatest increase in the number of people employed will be in the category of low-paying service occupations. This category, however, will not increase its share of total employment, whereas the category of high-paying service occupations will increase its share.

If the estimates above are accurate, which of the following conclusions can be drawn?

(A) In 1982 more people were working in low-paying service occupations than were working in high-paying service occupations.

(B) In 1995 more people will be working in high-paying service occupations than will be working in low-paying service occupations.

(C) Nonservice occupations will account for the same share of total employment in 1995 as in 1982.

(D) Many of the people who were working in low-paying service occupations in 1982 will be working in high-paying service occupations by 1995.（A）

(E) The rate of growth for low-paying service occupations will be greater than the overall rate of employment growth between 1982 and 1995.

这道题选项A是题目成立的必要而非充分条件。我们可以假设如果high occupation与low occupation都占50%时，比如以总数100人为例，各50人，low每增加一定数量时，只有high增加相同的数量才能使low占总数的份额不增加，如low增加10人，high必须也增加10人才可以使百分比都保持在50%，然而这是不可能的，因为题中说the greatest increase in the number of people employed will be in the category of low-paying service occupations，这说明low增加的人数要多于high，所以在各50人的情况下low的百分比(share)必然增加，与原题矛盾，所以各50人的情况不成立，而如果low小于50，high大于50的情况就更不成立，你可以自己试试。

也就是说如果题目成立，那么它必包含于A的范围内，但是A的范围并不仅仅有题目中所说的This category, however, will not increase its share of total employment, whereas the category of high-paying service occupations will increase its share这一情况，不知道你明白了吗

“也就是说如果题目成立，那么它必包含于A的范围内，但是A的范围并不仅仅有题目中所说的This category, however, will not increase its share of total employment, whereas the category of high-paying service occupations will increase its share这一情况，不知道你明白了吗”

就是这块不懂，请再讲讲，多谢

如果题目所说的成立，那么A肯定成立，但是根据A却不一定能推出题目中的条件：This category, however, will not increase its share of total employment, whereas the category of high-paying service occupations will increase its share.

比如，low为60，high为40，low增加了15（greatest)，high增加了5，总数变为120，但low的share从原来的60%变为了62.5%，还是增加了。当然这一点不用多想，只要知道A是题目成立的必要条件即可。