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B. It is happening today in the united states and europe.C. its happening today in most developing nations.D. Occured in the U.S and europe in the 19th century.

Explanation:

The demographic transition the movement of the population from a high birth and high death rate to low birth and low death rate of population growth.  It began in the 19th century in the economically developed nations of the world.  A large reduction in birth rates was followed by the early 20th century.  The developing nations of the world are having large-scale population growth while the developed are declining.

Answer (a) the itcz comes overhead for part of the year; i believe this is correct,plz check first; ///