What would be the effect of the following changes on the market price of a company's stock, all other things the same?

a$.$ Investors demand a higher required rate of return for stocks in general.

b$.$ The covariance between the company's rate of return and that for the market decreases.

c$.$ The standard deviation of the probability distribution of rates of return for the company's stock increases.

d$.$ Market expectations of the growth of future earnings $($and dividends$)$ of the company are revised downward.