（共5小题；每小题2分，满分10分）
根据短文内容，从下框的A---F选项中选出能概括每一段主题的最佳选项。 选项中有一项为多余选项。
A. Reasons for Having Large Families in Some Countries.
B. Calculation of Population Growth.
C. Efforts Made to Lower the Mortality Rate (死亡率)
D. High Fertility Rates in Developing Countries.
E. Low Fertility Rates in Developed Countries.
F. Birth Control Programs.
61. ________
Population growth is determined by the relationship between births and deaths. The growth rate is calculated by the birth rate minus the death rate. If the birth rate is 4%, for example, and the death rate is 3%, the population growing at rate of 1% per year.
62. ________
Historically, families had many children, but average life expectancies are low and many children died young. Improvements in nutrition and public health programs in the past 30 years have led to a drop in the mortality rate and hence to more rapid population growth.
63. ________
Public health programs and improved nutrition over the past 30 years have brought about declines in mortality rates in the developing nations. But fertility rates(出生率)have not declined as quickly, and the result has been high rates of population growth. Reduced population growth depends to some extent on decreased birth rates.
64. ______
Family planning and modern forms of birth control are important mechanisms for decreasing fertility, but by themselves such programs have had rather limited success in most countries where they have been tried. If family planning strategies are to be successful, they must make sense to the people who are supposed to benefit from them.
65. _______
To a great extent, in developing countries people want large families because they believe they need them. In some societies, children are important sources of farm labor, and they may thus make significant contributions to household income. In societies without any social security programs for the aged, children may also provide a vital source of income for their old parents.