单选题：Crop ProblemAdvances in technology have helped more of the w

Crop Problem
Advances in technology have helped more of the world's population live better andlonger -- and that is part of our problem!
Better health standards have kept larger number of people alive．The world’s population is now almost four billion and expected to double in twenty-five years．Growing population and slowly rising living standards have increased our need for food at the rate of 30million tons per year．As a result, the world's stockpile of food (食物储备) is decliningby about 10 million tons per year．
From the early I950s until 1972, world food production increased greatly．The GreenRevolution (绿色革命) extended scientific techniques to agriculture in the form of hybrid seed (杂交种) and poultry(禽类), chemical fertilizers (化学肥料) and pesticides(杀虫剂), and complex irrigation systems．Strains(品种) of corn (玉米), sorghum (高粱), soybeans（大豆）, wheat and rice were developed to flourish under particular climate and soil conditions．
In the United States, corn production rose to 110 bushels (蒲式耳：谷物、水果、蔬菜等容量单位相当于=36 368升,在美国相当于 35 238升) per acre from only 26 bushels per acre in the early 1900s．Milk production rose to 10 000 pounds per cow per year, compared with 600 in India, Chickens were bred to eat less, grow to maturity in shorter time, and produce more eggs．As a result of such scientific advances, our twelve Midwestern states alone now feed one fourth of the world’s people．
Crop disaster in 1972 brought an apparent end to the growth in production．Much of the extra yields had come from the use of chemical fertilizers, primarily petroleum based and now in short supply．The drop in world supplies of petroleum-based fertilizers is expected to cause a drop in crop yields of ten tons for each one ton decline in fertilizers applied．
This presents a particular problem for underdeveloped nations that often lack the foreign exchange necessary for buying fertilizer．The problem is so severe that Philip Handler, president of the National Academy of Sciences, has predicted one million child deaths per month in these nations by the year 2025．The expected world population will reach______in 25 years． A．2 000 000 000
B．4 000 000 000
C．6 000 000 000
D．8 000 000 000