Why Did the "Long Peace" Occur

Why did the "Long Peace" occur?

A) Germany reunited and created a balance of power with France until just before World War I.
B) The United States returned to its isolationist strategy and did not start new wars.
C) The Soviet Union recognized that the United States was much stronger and made significant concessions to the West.
D) The fact that just the United States and the Soviet Union dominated international relations meant miscalculations were less likely.
E) The alliance system was less institutionalized,meaning that European powers could shift allegiances when necessary to protect their interests.