Which of the following is a source of market risk?

a. Holding stocks in many companies carries the risk of a reduced average return.
b. Real GDP varies over time and sales and profits move with real GDP.
c. When a paper producer has declining sales, it is likely that so will other paper producers.
d. If stockholders become aggravated with the way a CEO runs a company, the price of that company's stock might fall in the stock market.