D is your answer .
Explanation: As the market rose through the 1920s, stockbrokers allowed investors to buy "on the margin," that is, by making only a small down payment, often as low as 10 percent. Easy availability of credit lured many investors to make quick profits on borrowed money. This credit expansion created a speculative bubble that inflated stock prices to unrealistic highs. The bubble burst in October 1929 when the British raised their interest rates to attract capital invested in the US market. As stock prices started falling, many investors who had bought on credit failed to repay, so brokers sold stocks held as collateral. This chain reaction led the market into a downward spiral.
