What made americans so willing to engage in stock market speculation in the 1920s?

The main reason why so many Americans were so willing to engage in stock market speculation in the 1920s is because the "roaring" economy of the 20s meant that investors could make wildly high returns on their investment in relatively small amounts of time, especially since there was practically no government regulation of investment under the Great Depression, which began with the crash of 1929. 

The answer should be d.