Final answer:
Shanghai's population remains stable due to migration from rural areas, while Tokyo faces decline due to low birth rates and lack of similar internal migration, coupled with Japan's stage of economic development prompting negative population growth.
Step-by-step explanation:
The direct answer to why Shanghai isn't facing the potential population decline that Tokyo is can be attributed to B. China has migration from rural areas to Shanghai maintaining its population.
Despite the one-child policy in China being in effect from 1979 to 2015, Shangai's population has remained stable due to the influx of migrants from the country's rural areas. This internal migration has helped to offset the population control measures. In contrast, Japan has not seen a similar internal migration trend, and its population, including that of Tokyo, is aging due to low birth rates and high personal incomes leading to smaller family sizes. With a replacement level of about 2.1 children per couple to maintain a stable population and Japan's average family size being quite low at about 1.2 children, Tokyo is now facing a decline in its population. Additionally, while the one-child policy was phased out, because of its long-term effects and a cultural preference for male heirs, population dynamics in China are complex with the impacts still being assessed today. Tokyo's situation is further compounded by economic development reaching a stage where negative population growth begins.