4. Consider now a seller whose marginal cost for selling successive units of the good is as follows. 9 MC(q) 0 1 $1 2 $2 3 $6 4 $9 where q is quantity and MC(q) is the marginal cost. (a) On the graph drawn in part 2, sketch the seller's marginal cost as a function of quantity. Clearly label your graph. (b) What quantity leads to efficient trading?