Four modernizaron is the correct answer
It was a chain reaction. Long story short, the countries were pulled into the war because of agreements they had with their allies. This treaty is what caused the countries to enter WWI, to protect and defend their allied countries who were already in the war.
Credibility of the source is checking the degree of authenticity of a source
(You want to know how reliable is this information?)
There are multiple factors that can help to check its credibility:
Is the author of the source trusted?
Is the background of the source safe?
Is it a primary or secondary source?
Have the source become unaccurate or
obsolete due to time?
Is there any motive or signs that the source is biased?
I think it is some city in china
