Final answer:
The stock market crash of 1929 was directly caused by speculation on stocks and buying on a margin.
Step-by-step explanation:
The correct answer is C. speculation on stocks and buying on a margin. Speculation in the stock market and buying stocks on margin were common practices during the 1920s, leading to a speculative bubble and an artificially inflated stock market. When the bubble burst and stock prices plummeted, it resulted in the stock market crash of 1929.
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