Link1)1- whenever there is a positive externality in the market this means that the social benefit is higher than the private benefit but when private benefit is equated to the marginal cost the outcome that is produced is less than the socially effecient output.

so answer is D and c

2- open access resources are the ones those are available to the general punlic for use , now these are generally the ones those are overexploited like the grazing fields those are available to the farmers here each farmer tries to maximize his outcome rather than thinking of the cost of his grazing cow grazing , this way when each farmer tries to equate their costs and benefits individually they forget about the social cost and thus overexploitation.

this is also known as tragedy of commons

thus answer is A and C
Link2)Social Marginal cost associated with the fireworks = MCa +MCb

MSC= 10+Q+10+3Q

MSC=20+4Q

MBa = 70-Q

At the socially optimal level of fireworks:

MSC= MBa

20+4Q = 70-Q

5Q = 50

Q = 10

Thus the socially optimum level of fireworks = 10