[单项选择]It is postulated that population trends have an effect on economic fluctuations.
A. challenged
B. assumed
C. deducted
D. decreed

[单项选择]It is postulated that population trends have an effect on economic fluctuations.（　）
A. deducted
B. assumed
C. decreed
D. challenged

[填空题]Since the early 1990s environmental trends have started to affect our economic trends, with the effects of losing large amounts of topsoil being felt.

[简答题]population ageing

[简答题]
1 Population ecology is the science that measures changes in population size and composition and identifies the causes of these fluctuations．Population ecology is not concerned solely with the human population．In ecological terms．a population consists of the individuals of one species that simultaneously occupy the same general area．rely on the same resources, and are affected by similar environmental factors．The characteristics of a population are shaped by its size and by the interactions among individuals and between individuals and their environment．
2 Population size is a balance between factors that increase numbers and factors that decrease numbers．Some factors that increase populations are favorable light and temperature，adequate food supply, suitable habitat, ability to compete for resources, and ability to adapt to environmental change．Factors that decrease populations are insufficient or excess

[单项选择]
1 Population ecology is the science that measures changes in population size and composition and identifies the causes of these fluctuations．Population ecology is not concerned solely with the human population．In ecological terms．a population consists of the individuals of one species that simultaneously occupy the same general area．rely on the same resources, and are affected by similar environmental factors．The characteristics of a population are shaped by its size and by the interactions among individuals and between individuals and their environment．
2 Population size is a balance between factors that increase numbers and factors that decrease numbers．Some factors that increase populations are favorable light and temperature，adequate food supply, suitable habitat, ability to compete for resources, and ability to adapt to environmental change．Factors that decrease populations are insufficient or excessive
A. Any species of life can be studied in population ecology．
B. Population ecologists care about the future of humanity．
C. The growth of the human population is a major concern．
D. Population ecology does not consider humans worthy of study．

[单项选择]It is postulated that a cure for the disease will have been found by the year 2020.（　）
A. challenged
B. assumed
C. deducted
D. decreed

[单项选择]In （）of expert help, we have to rely on our own efforts.
A. default
B. fault
C. defense
D. defiance