The Ranks of the Middle-Income Countries Expanded Significantly Between 1999

The ranks of the middle-income countries expanded significantly between 1999 and 2000.Why?

A) The global grain supply increased dramatically.
B) China was reclassified from low to middle income because of its economic growth.
C) There was a general shift of buying power from the global North to the global South.
D) The United States was reclassified to middle income due to the decline of its middle class and living standards more generally.