1.何謂外部性(externality)。(5分)

2. Given sorting problem µ convex hull，which one of the following statements can be derived ?(Choose one) (1) The Lower Bound of convex hull is W(n log n). (2) The Lower Bound of convex hull is W(n). (3) The Lower Bound of Sorting is W(n log n). (4) The Lower Bound of Sorting is W(n). Please explain how to find the lower bound by problem transformation. (20%)