Can someone give me a summary of what was the great depression and why was it so important?

The Great Depression was the worst economic downturn in the history of the industrialized world, lasting from 1929 to 1939. It began after the stock market crash of October 1929, which sent Wall Street into a panic and wiped out millions of investors.

If you have choices which can be:
A. Johnson lost interest in the issue of Reconstruction upon Lincoln's assassination.
B. Johnson became more interested in the passage of the Thirteenth through Fifteenth Amendments. 
C. Johnson believed that the Union was paramount and the South should be represented.
D. Johnson did not believe Southern landowners had done anything wrong leading up to the war.
E. Johnson wanted to show his authority by passing the Civil Rights Bill of 1866

Then the answer would be letter E. President Johnson drifted from his original plan which was to punish the southern Aristocrats from reentering the Union and it is all because of he wanted them to pass the Civil Rights Bill of 1866 which is from his authority.