Which one is correct?

why did the stock market crash so quickly?

a) it crashed because people sold off stocks whose prices were getting very low and no one would buy more.

b) it crashed because the prices of stocks were too high to start with and people got tired of the whole business.

c) it crashed because people were out of work and hungry.

d) it crashed because people made a lot of bad investments and they stopped buying.

e) it crashed because that is what the stock market does every ten years.