A problem for people who bought stock on credit during the 1920s was that if the stock market collapsed, they

Buying stock on credit, or on margin, meant borrowing money to pay for part of the purchase. If stock prices collapsed, the shares could be worth far less than the borrowed amount, but the buyer still had to repay the loan. Thus, buyers could owe more than they could repay; the other choices do not describe the debt risk.

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