Civilization has changed much in the last 2,000 years, or two
a. millennia b. centuries c. millennium d. decades

A.      Millennia
Millennia a plural form of millennium is equivalent to 1000 years. Which means that 2 millennia is equivalent to 2,000 years span of time. Centuries, a plural form of century on the other hand is 100 years in total, only. And lastly a decade, is 10 years in total. All of these are terms in which they pertains to certain amount of time which increases by power of 10. As you noticed. 10 – Decade 100 – Century 1000 – Millennium