Assume that a risk-averse investor owning stock in Miller Corporation decides to add the stock of either Mac or Green Corporation to her portfolio. All three stocks offer the same expected return and total risk. The covariance of return between Miller and Mac is =.05 and between Miller and Green is =.05. Portfolio risk is expected to:
a. Decline more when the investor buys Mac.
b. Decline more when the investor buys Green.
c. Increase when either Mac or Green is bought.
d. Decline or increase, depending on other factors