What two factors largely contribute to a stock market crash? $($You will get $50\%$ of the mark for each correct answer, and $-25\%$ of the mark for each incorrect answer$).$

a$.$ Panic selling and loss of investor confidence.

b$.$ Strong regulatory frameworks.

c$.$ Speculative investments and overvaluation.

d$.$ Tightening of credit markets.

e$.$ High levels of employment.

f$.$ Sudden economic growth.