3．集合概念推理
集合概念推理也可以称之为百分比推断，它指的是在一个集合中，包含两个相对的方面，共同构成一个大的集合。一方所占的比例减少意味着另一方所占比例的增加；反之，一方所占的比例增加意味着另一方所占比例的减少。根据这一思路，可以由集合中一方的数量变化推断另一方的消减。
例一：
A folk culture is small, isolated, cohesive, conservative, nearly self-sufficient group that is homogeneous in custom and race, with a strong family or clan structure and highly developed rituals. …Unaltered folk cultures no longer exist in industrialized countries such as the United States and Canada….
By contrast, a popular culture is a large heterogeneous group, often highly individualistic and constantly changing.
What does the author imply about the United States and Canada?
(A) They value folk cultures.
(B) They have no social classes.
(C) They have popular cultures.
(D) They do not value individualism.
例二：
Both the number and the percentage of people in the United States involved in nonagricultural pursuits expanded rapidly during the half century following the civil war, with some of the most dramatic increases occurring in the domains of transportation, manufacturing, and trade and distribution.
What can be inferred from the passage about the agricultural sector of the economy after the Civil War?
(A) New technological developments had little effect on farmers.
(B) The percentage of the total population working in agriculture declined.
(C) Many farms destroyed in the war were rebuilt after the war.
(D) Farmers achieved new prosperity because of better rural transportation.
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