The correct answer is:
A. were each connected to alliances that included many world powers.
Explanation:
Austria-Hungary's ultimatum to Serbia led to a far-reaching conflict because both countries were part of alliances that included other powerful nations. When Austria-Hungary issued the ultimatum, it triggered a chain reaction due to the alliance system in place at the time. Serbia was supported by Russia, while Austria-Hungary had the backing of Germany. This led to a domino effect where other countries were drawn into the conflict, eventually escalating into World War I. The interconnected alliances played a crucial role in turning a regional conflict into a global war involving many world powers.