Question: People commuting to a city can choose to go either by bus or by train. The number of people who choose either method depends in part upon the price of each. Let f(P1, P2) be the number of people who take the bus when P1 is the price of a bus ride and P2 is the price of a train ride. What can you say about the signs of ∂f /∂P1 and ∂f /∂P2? Explain your answers.