II. 簡答題 (共 10 分) Lake Union has a polluting firm on its shore. For every toy that the firm produces, it creates some pollution to the lake. The market price for the toy is $100. The private costs of production and the external costs created by pollution are listed in the following table: | Unit produced | Marginal Private Costs | Marginal water pollution | | | | | | 1 | $50 | $30 | | 2 | $60 | $30 | | 3 | $70 | $30 | | 4 | $80 | $30 | | 5 | $90 | $30 | | 6 | $100 | $30 | | 7 | $110 | $30 |