Sean Can Build Sandboxes Twice as Fast as Matt

Multiple Choice

Sean can build sandboxes twice as fast as Matt. Sean can assemble swing sets three times as fast as Matt. The law of comparative advantage suggests that

A) Sean can gain only at Matt's expense.
B) Matt can gain only at Sean's expense.
C) both can gain if Sean specializes in assembling swing sets and Matt in building sandboxes.
D) both can gain if Sean specializes in building sandboxes and Matt in assembling swing sets.