$5.$ Correcting for negative externalities $-$ Regulation versus tradablepermits

Method $2$: Tradable Permits$$

Meanwhile, the other employee proposes using a different strategy to achieve the government's goal of reducing pollution in the area from $12$ units to $6$ units. This employee suggests that the government issue two pollution permits to each firm. For each permit a firm has in its possession, it can emit $1$ unit of pollution. Firms are free to trade pollution permits with one another $($that is$,$ buy and sell them$)$ as long as both firms can agree on a price. For example, if firm A agrees to sell a permit to firm B at an agreed$-$upon price, then firm B would end up with three permits and would need to reduce its pollution by only $1$ unit while firm A would end up with only one permit and would have to reduce its pollution by $3$ units. Assume the negotiation and exchange of permits are costless$.$

Because firm C has high pollution$-$reduction costs, it thinks it might be better off buying a permit from firm B and a permit from firm A so that it doesn't have to reduce its own pollution emissions. At which of the following prices are both firm B and firm A willing to sell one of their permits to firm C$?$Check all that apply.