Engineers and physicists dream of solving the world's energy supply problem by constructing power plants that would convert hydrogen to helium. our sun seems to have solved this problem. choose the statement below that correctly identifies a major obstacle to this potentially environmentally clean mode of energy production.
a. it is difficult to achieve the temperatures of the interior of the sun.
b. there is not enough hydrogen.
c. the helium shortage is driving up prices.
d. all the hydrogen is locked up in water already.

d. stage 4

explanation:

the existence of some kind of demographic transition is widely accepted in the social sciences because of the well-established historical correlation linking dropping fertility to social and economic development.[1] scholars debate whether industrialization and higher incomes lead to lower population, or whether lower populations lead to industrialization and higher incomes. scholars also debate to what extent various proposed and sometimes inter-related factors such as higher per capita income,lower mortality, old-age security, and rise of demand for human capital are involved.[2]

answer(c) carved on a babylonian clay tablet that showed earth as a flat circular disk;