Tensions Prior to WWI

We discussed 4 tensions that helped cause WWI. These were:  Military/Arms Race, Economic Rivalry, Alliance Systems, and Nationalism.  Which one or ones of these do you believe were the MOST significant in starting the war.  Explain why you think this.

Life in Early 20th Century

We've only just begun our course on 20th Century History.  In your opinion, what would have been 2 exciting elements of living during the early 1900s?  What would have been 2 challenges?