Answer:
B
Step-by-step explanation:
The assassination of Franz Ferdinand stoked old tensions beyond the Balkans. The crisis spread as other powers pledged support for either Austria or Serbia. Austria knew that conflict with Serbia would likely involve Russia, which saw itself as Serbia's protector.
Officially, World War I started on July 28th when Austria-Hungary declared war on Serbia. In response, Russia mobilized its forces further on July 30th and began to prepare for war with Austria-Hungary.