Consider two firms with the following marginal abatement costs (MAC) as a function of emissions (E):

MAC_1 = 11 - 2E_1

MAC_2 = 12 - E_2,

and assume marginal external damages (MED) from aggregate emissions from the two firms (E_Agg = E_1 + E_2) is:

MED = E_Agg.

The socially efficient level of aggregateabatementis ____.