The principal factor depressing life expectancy in developing countries has always been the high death rate for infants and children. The World Bank studies suggest that as much as two thirds of the difference in life-spans between people in developed countries and those in developing ones can be traced to differences in survival rates for children under five. It is here where the most improvement has come. According to UN estimates, significant regional drops in infant mortality - ranging from 25 percent to 60 percent and centering near 40 percent - appear to have taken place between the late 1950s and the late 1970s in northern Africa, Latin America, and Asia. Although sub-Saharan Africa' s mortality trends cannot be quantified with confidence, there is reason to believe that life expectancy has risen and infant mortality has declined in that region as well. There is little doubt that population growth has accelerated in sub-Saharan Africa since the 1950s; in fact, sub-Saharan Africa is widely thought to have the highest rate of population growth of any major region in the world. Only a small portion of that acceleration is likely to have been caused by increases in fertility (and increases in fertility, insofar as they have occurred, may also imply improvements in health and nutrition).
Mortality, of course, is not a perfect measure of nutritional change. Improved nutrition is only one of a number of forces that have been pushing down death rates in developing countries. Others include the upgrading of hygiene and sanitation; the extension of public health services; medical innovations; improvements in education, communications, transportation, and, in some areas, civil order. Even so, the extent to which improvements innutrition—both direct and indirect—have reduced mortality in developing countries has frequently been underestimated. For example, Sri Lanka experienced an abrupt jump in life expectancy shortly after the Second World War. Whereas this was long described as a "technical fix"—a triumph of DDT over the anopheles mosquito—years later researchers realized that abrupt and rapid drops in mortality had also taken place in Sri Lanka' s highlands, or "dry zones", where malaria had never been a serious problem. In both highlands and lowland regions health improved in tandem with access to food.
According to the author, life expectancy in developing countries is not high mainly because developing countries______.
A．have a low standard of living
B．have no public health services
C．have no public and private hygiene and sanitation
D．have low survival rates for infants and children