What is the decisive factor in future competition between nations

A．Wood-bunting stoves are safer and more efficient than they used to be.
B．No factories exist for the purpose of producing dirty wood chips.
C．Both catalytic combustors and wood agitators save money in the long run.
D．Wood-burning stoves with catalytic combustors cost between $ 100 and $ 200.

A．flexibility
B．diversity
C．variety
D．multiplicity