After the Archduke's assassination, all the great powers of Europe began declaring war on one another. This is an example of which idea:
A. Alliance systems
B. Militarism
C. Imperialism
D. Nationalism

B. Militarism

Explanation:

Through the process of elimination:

Alliances are based on certain powers helping each other, nationalism is a strong sense of loyalty and pride for one's nation, and imperialism is the act of conquering a land to expand an empire. :)

The concept of militarism:

Basically a strong sense that a group of people or nation should establish and maintain a strong military, which is used for the nation's interests, war, defense, and more.

john maynard keynes developed new economic theories in response to the : b) great depression of the 1930s. his theories are so important that they are known as "keynesianism". in his works he supports the idea of a government with deeper involvement and intervention to end the great depression.