Answer:
D. Russia had assumed a two-front war against Germany and Austria-Hungary, and Germany had assumed a two-front war against Russia and France.
Step-by-step explanation:
There are 2 main sides during the world war I.
The Central powers which consist of Germany, Austria-Hungary, ottoman empire, and Bulgaria and the Allied powers which consist of Serbia, France, Russia, British Empire , and Belgium.
Germany, Austria-Hungary, Russia , and France were among the strongest country that are really close to each other borders. It is very easy for their military united to initiate attacks from land. (United Kingdom is located on a separate island).
Because of this, they anticipated two-front wars in which Russia had assumed a two-front war against Germany and Austria-Hungary, and Germany had assumed a two-front war against Russia and France.