Which of the following is the geometric mean of 4 and 9?

Congress passed the Clean Air Act in 1970. Since this act was passed, emissions of the six main air pollutants

A) have increased significantly due to the growth of the U.S. economy. B) cannot be measured since Congress failed to appropriate money to monitor the level of emissions. C) have fallen by more than one-half. D) have remained essentially constant, even though significant economic growth has occurred in the United States since 1970.

In a Bertrand model with differentiated products

A) firms can set price above marginal cost. B) firms set price at marginal cost. C) price is independent of marginal cost. D) firms set price independently of one another.