Answer:
D. An alliance system that required allies to aid one another in armed conflicts
Explanation:
The alliance between the UK, France and Russia became known as The Triple Entente, a military alliance that opposed the Union between Germany Austro-Hungary and Italy.
Some years before, a key alliance between France and Russia called the Dual Alliance opposed an increasingly powerful German - Austrian influence in Central and East Europe, as well as in the Balkans.
The alliance system can be compared to a entangled chess game where , two groups stand against, Central powers, against Allied or Entente. In this complex system any move would provoque a reaction of the neighbouring states, so a cascade followed as a reaction to every minuscle event.
The alliance system was one of the main causes since reactions to local events had a multinational event and made the parties act in response.
