What Was a Key Reason Why the Years 1820-1845 Were

What was a key reason why the years 1820-1845 were relatively calm on the European continent, compared to the period from 1789 to 1815?

A) The European states were on nearly equal footing, without a specific group considered to be the great powers.
B) The great powers rarely disagreed because each of them rejected monarchical authority.
C) Following the Congress of Vienna, the great powers sought to use diplomacy to help avoid war.
D) The great powers remained uninvolved in conflict unless it took place in their particular country.
E) The desire to build overseas empires had died out, allowing European countries to focus on domestic concerns.