请选择（47）处的最佳答案．

A．speeded up

B．put off

C．Slowed down

D．stopped

A．Right

B．Wrong

C．Not mentioned

B．started

C．kept

D．stopped

A．Right

B．Wrong

C．Not mentioned

Natural Selection

Natural selection describes the biological process in which the differences of individu-als within a population influence their abilities to survive and reproduce in an environment.The differences in individuals are a result of their genetic inheritance from their parents.In a population, any characteristic which blocks reproduction tends to decrease generation by generation.In time, the ill-adapted die out.On the other hand, the individuals who do survive and reproduce will tend to produce offspring(后代which are better adapted to the environment.Natural selection tends to promote adaptations that will increase the organ- ism's(生物体)ability to survive in an environment.

Natural selection can serve to stabilize a population if the new traits, called mutations (突变), are eliminated when they appear because they are not as well-adapted to the envi-ronment.The opposite effect is' obtained when a new trait is introduced which allows indi-viduals to adapt better over time, the species will change as this mutation becomes more widespread in the population.In human beings, increased brain size helped individuals to adapt better; and so brain size increased gradually in the species.Changes in the overall genetic makeup of a population are normal when there are environmental changes, espe-cially severe environmental disruptions.Specialized adaptations to specific environments can lead, over time, to the development of subpopulation of individuals, who are better a- dapted to particular soil conditions, food sources and so on.Given enough time, these subpopulations may develop into separate species, such as zebras(斑马)and horses, living in distinct environments and not interbreeding.

第 31 题 Individuals in a population differ from one another because__________.

A．they have different success generation by generation

B．they are ill-adapted and eventually die out

C．they have different genetic inheritances

D．they want to survive and reproduce successfully

A．where fewer people die from heart problem．

B．whether they have the risk of heart attack，heart failure or stroke．

C．would probably have lower rates of heart disease．

D．how to test a person’s NT—pmBNP level in the blood by himself．

E．his heart muscle would be under pressure in some way．

F．that napping is of great benefit to women too．

A．continuing

B．declining

C．occurring

D．responding

How much change has really occurred in the American family and what are the implications of these changes？ First, the household size has changed greatly since 1790. From 1790 to 1978 the mean family size was cut in half from 5.79 persons to 2.81 persons. In 1790 almost 63 percent of all persons lived in the households of five or more people. By 1978 the size accounted for a little over 14 percent of all households.

By the end of the 19th century a majority of Americans were living in urban areas, and the family was very much influenced by the rapid development of industrialization. With the arrival of immigrants, the urban population was increasingly heterogeneous(由不同成分组成的). This challenges the exclusiveness of any single family pattern.

In the 20th century, the ideal American family consisted of a husband and wife living with dependent children. They lived in a household of their own provided for by the husband's earnings. The wife was responsible for emotional maintenance of the marriage and for raising the children and running the household.

The major change in the family in this century has been due to married women entering the work force. This, at least for periods of time. has taken the woman out of her full-time involvement in the home. Of all husband and wife families about 40% have both in the work force at any given time.

In the past the identity of the individual was submerged in the family. In general, reputation in the community came from the family. Today, however, whatever individuals achieve is usually assessed on its own merit, and family has little relevance. Individuals make it or don't make it essentially on their own.

A sociologist describes another way in which the American family has changed. Today, in the Western world, the major burdens that are a part of the family system are emotional ones. But in the 19th century the family was much more involved with economic needs and tasks; family and relatives were valued for providing assistance during crisis.

What can we infer from the figures about the American household size in the first paragraph？

A．In the 20th century, there are some families without children.

B．Most of the families in the 19th century had five or more children.

C．More and more Americans tend to live with their parents after marriage.

D．Fourteen percent of the U.S. families now have five or more children.

B．live

C．survive

D．come