What weakened many of the ancient cities leaving them vulnerable to nomads?

What would lead a historian to consider two events to have a relationship of correlation rather than causation?

By the end of the 1800s, there was a change in the nature of the immigrants coming to the united states. immigration from northern europe—england, ireland, germany, and scandinavia—declined, while immigration from italy, russia, and austria-hungary grew. in fact, between 1900 and 1910, more than three-fourths of all immigrants came from these three countries. unfortunately, these countries were far removed england, the home of our founders, and the immigrants who came from them had a hard time adjusting to life in their new country.

Which statement best describes the response by the united states and its allies to the berlin blockade? planes dropped tons of supplies around the city each day. trucks smuggled tons of supplies into berlin about once a week. tons of supplies were delivered to west berlin by boats. tons of supplies were purchased from east berlin each day.