What isthe definition of infertility according to the demographers?

A．demographersB．taxonomistsC．chronologistsD．psychologists A: demographers B: taxonomists C: chronologists D: psychologists

It is a trend that shows no sign of slowing. Demographers (人口统计学者) reckon that three-quarters of humanity could be city-dwelling by 2050, with most of the increase coming in the fast-growing towns of Asia and Africa.(CET6-2015.6)