How would I go about solving this problem? The answer is D but I am not sure how to get to It.

5. The next two questions (5 and 6) concern a competitive market with an externality in production and no externality in consumption. Quantity Demanded is given by Q=100-2P and Quantity Supplied is given by QS=P-20. Assume the marginal social cost of production is MCS=30+2Q The efficient quantity is while a competitive market produces a. 10:20 b. 20:10 c. 10;8 d. 8;20