Historically, as countries have developed industrially, they have undergone declines in death rates
followed by declines in birth rates. Over time they have tended to move from rapid increases in population to slower increases, then to zero growth and finally to population decreases. The model which
demographers use to help explain these changes in population growth is known as the demographic
transition model. In order to properly appreciate the demographic transition model, it is necessary to
understand two basic concepts: the crude birth rate (CBR) and crude death rate (CDR). The CBR is
determined by taking the number of births in a country in a given year and dividing it by the total population of the country and then multiplying the answer by one thousand. So, for example, the CBR of the United States in 2004 was 14 (in other words, there were 14 births per thousand living people in that year). CDR is worked out in a similar way. The CDR for the United States in 2004 was 8 per thousand.
      The first stage of the demographic transition model portrays a preindustrial era when both the birth
rate and the death rate were high. Typically, women gave birth to a large number of babies. This was partly due to cultural and religious pressures but also because families required a large number of children, since often many didn’t survive into adulthood due to the harsh living conditions. Furthermore, children were needed to help adults work the land or perform other chores. The death rate was high due to the high incidence of diseases and famine and also because of poor hygiene. Total population tended to fluctuate due to occasional epidemics, but overall there was only a very gradual long-term increase during this stage.
      During the second stage, improvements in hygiene, medical care, and food production led to a decrease in the death rate in newly industrializing regions of Western Europe. However, birth rates remained high due to the tradition and because many people were involved in agrarian occupations. The combination of a lowered CDR and a stable CBR led to dramatic increases in population starting at the beginning of the nineteenth century.In stage three, birth rates also began to fall.   
(A)   In cities there was less incentive to produce large numbers of children, since city dwellers no longer worked the land, and the cost of raising children in an urban environment was greater than in rural districts.   
(B)   Furthermore, more children survived into adulthood due to improved living conditions.   
(C)   These economic pressures led to a lower CBR and over time the numbers of people being born started to approximate the numbers dying.   
(D)  
      The final stage, which some demographers have called the postindustrial stage, occurs when birth rates and death rates are about equal. In this case there is zero natural population growth. Over time the birth rate may fall below the death rate, and without immigration the total population may slowly decrease. By the early twenty-first century, several European countries were experiencing population declines due to the CDR outstripping the CBR. For example, in Italy in 2004 there were about 9 births per thousand against 10 deaths per thousand.
      The demographic transition took about 200 years to complete in Europe. Many developing countries
are still in stage two of the demographic transition model: births far outstrip deaths. In these countries, CDR has declined due to improvements in sanitation and increases in food productivity, but the birth rate has still not adjusted downward to the new realities of improved living conditions. This imbalance of births over deaths in the developing world is the fundamental reason for the dramatic population explosion in the latter half of the twentieth century. However, population statistics indicate that in many less developed countries the CBRs have begun to decline over recent decades, giving rise to optimism in some quarters about future trends. The rapid industrialization of many parts of the developing world has meant that these countries have reached stage three of the model much faster than countries in the developed world did during the nineteenth century. This fact has led many demographers to predict that world population will reach an    equilibrium    level sooner and at a lower total than more pessimistic earlier predictions.
【題組】8. The word “it” in paragraph 1 refers to ______ .
(A) population
(B) year
(C) country
(D) number