补全短文
A. the next half century B. reducing death rates C. developing world D. infectious diseases Phrases: A. could potentially become the toughest problem of【T13】______ B. provides the tools and methods for lengthening life spans and【T14】______ C. with virtually all the new growth occurring in the【T15】______ D.【T16】______are much less frequent due largely to public health measures It is the application of technology to physical resources that provides for human life and eventually sets the limits on the number of people who can be fed, but technology also【T17】______As a result, high birth rates are no longer closely matched by high death rates as they were until modern times:【T18】______: and physical vitality has been increased by improved nutrition. These changes have brought about what is today familiarly called the "population explosion" which, unless checked,【T19】______If the current rate of population growth continues in some parts of the world, the world population is expected to reach 9. 1 billion by 2050,【T20】______while the total population of richer countries is expected to remain largely unchanged at 1. 2 billion.