Which sentence uses the time-related term correctly?

a. jill enjoys reading about american history, especially the last 40 years, or four decades, of the nineteenth century.

b. jill enjoys reading about american history, especially the last 40 years, or four millennia, of the nineteenth century.

c. jill enjoys reading about american history, especially the last 40 years, or four eras, of the nineteenth century.

d. jill enjoys reading about american history, especially the last 40 years, or four centuries, of the nineteenth century.