Which diagram best represents an important part of eastern European history?

A.
Two boxes are connected by a unidirectional arrow. Box1:Serbia attacks the Austro-Hungarian Empire.Box2:The Soviet Union leaves eastern Europe.
B.
Two boxes are connected by a unidirectional arrow. Box1:World War II begins.Box2:The Austro-Hungarian Empire conquers eastern Europe.
C.
Two boxes are connected by a unidirectional arrow. Box1:The Soviet Union collapses.Box2:Democracy spreads in eastern Europe.
D.
Two boxes are connected by a unidirectional arrow. Box1:World War I ends.Box2:Serbia becomes the most powerful state in eastern Europe.