In the 20th century the planet's population'doubled twice.It will not double even once in the 1 century,because birth rates in much of the world have 2 steeply.But the number of people over 65 is set to 3 within just 25 years.This shift in the structure of the population is not as momentous as the 4 that came before.But it is more than enough to reshape the world economy.5 the UN's population 6,the standard source for demographic estimates,there are around 600m people aged 65 0r older 7 today.That is in itself remarkable;the author Fred Pearce claims it is 8 that half of all the humans who have ever been over 65 are alive today.But 9 a share of the total population,at 8%,it is not that 10 to what it was a few decades ago.By 2035,11,more than l.1 billion people-13%of the population-will be above the age of 65.This is a 12 result of the dropping birth rates that are slowing overall population growth;they mean there are 13 fewer younS people around.The"old-age dependency ratio"-the ratio of old people to those of working age-will 14 even faster.In 2010 the world had 16 people aged 65 and over for every 100 aduILs between the ages of 25 and 64,15 the same raLio it had in 1980.By 2035 the UN 16 that number to have risen to 26.In rich countries it will be much higher.Japan will have 69 0ld people for every 100 0f working age by 2035,Germany 66.17 America,which has a relaLively high 18 rate,will see its old-age dependency rate rise by more than 70%,t0 44.Developing counLries,19 today's ratio is much lower,will not see absolute levels rise that high;20 the proportional growth will be higher.Over the same time period the old-age dependency rate in China will more than double from 15 t0 36.Latin America wiU see a shift from 14 to 27.18选？
A.fertility
B.marriage
C.mortality
D.divorce