$($C$)$ Which of the following statements is INCORRECT?

a$.$ The portfolio risk is generally the average of the stocks' standard deviations.

b$.$ The beta of a portfolio is the weighted average of each of the stock's betas.

c$.$ Diversification is completely useless for reducing the risk if the stocks in the portfolio are perfectly positively correlated.

d$.$ Diversification can be advantageous when the correlation between assets is less than $1,$ potentially reducing portfolio risk.

e$.$ The expected return on a portfolio is the weighted average of the expected returns of the individual assets in the portfolio.