Refer to Figure 4.1. A Nash equilibrium can be found at

Which of the following is(are) the characteristics of a bubble

a. Bubbles emerge when investors disagree about the particular economic importance of an event b. Bubbles involve very large increases in trading volume c. Bubbles may continue even when many expect a bubble and doesn't pop until a sufficient number of skeptical investors act simultaneously d. All of the above

In the presence of positive externalities, a free market will choose a price which is too ____ and produce an output which is too ____ compared with the social optimum.

A. high; low B. low; low C. high; high D. low; high