If a product has zero external costs, then marginal social cost equal marginal private cost

The table above gives Cathy's total utility from Mt. Dew. Cathy's marginal utility from the 3rd Mt. Dew is ________ units

A) 75 B) 65 C) 25 D) 20

Suppose your expenses for this term are as follows: tuition: $10,000, room and board: $6,000, books and other educational supplies: $1,000

Further, during the term, you can only work part-time and earn $8,000 instead of your full-time salary of $20,000. What is the opportunity cost of going to college this term, assuming that your room and board expenses would be the same even if you did not go to college? A) $11,000 B) $17,000 C) $23,000 D) $29,000