According to Meadows, Meadows, and Randers, Which of the Following

According to Meadows, Meadows, and Randers, which of the following is necessary to allow for a sustainable world throughout the twenty-first century?

A) every couple in the world limiting itself to two children
B) every nation in the world providing a standard of living for its citizens that is equivalent to that enjoyed in the United States
C) the development of fusion energy
D) every nation in the world adopting the energy consumption practices of the United States