Which of the following statements is CORRECT?

a. Diversification does not reduce the riskiness of a portfolio of stocks.

b. Risk-averse investors require lower rates of return on investments whose returns are highly uncertain.

c. An individual stock's diversifiable risk can be lowered by adding more stocks to the portfolio in which the stock is held.

d. The tighter the probability distribution of its expected future returns, the lower the risk of a given investment as measured by its standard deviation.