18th or 19th centuries 为什么用复数?而不是18th or 19th century.

题目

18th or 19th centuries 为什么用复数?而不是18th or 19th century.
如题

答案

[1] The Industrial Revolution of the late 18th and early 19th
centuries 正确 was revolutionary because it changed -- revolutionized -- the
productive capacity of England,Europe and United States.
[2] People living in the 18th or 19th [century正确] would never imagine the advanced household appliances we are now able to enjoy.