What has been the most common reason for one-party losing control the national government

In the electoral history of  the US and other democratic countries, except for unusual circumstances or events, the economy is the main concern of voters. People are more concerned about having jobs, getting incomes to raise kids and keep families, having the possibility of buying a home, and so on. Normally, people are less worried about events in foreign countries.  

Explanation:

can someone me with this question:  

1) the end of the conflict between the north and the south didn’t just mark the end of a war; it marked the end of an entire way of life.” which statement best describes what this statement means?  

a) the southerners were no longer free and were controlled by the northern states.

b) the industrial economy in the north was not sustainable.

c) slavery became illegal, which changed completely the south’s labor force and the agricultural economy was destroyed.

d) the states decided to relinquish their independence and return to britain’s rule.