Substitution occurs when firms replace one input for another, as when a farmer uses tractors rather than labor when wages rise. Consider the following changes in a firm's behavior. Which represent substitution of one factor for another with an unchanged technology, and which represent technological change? Illustrate each with a graphical production function.
a. When the price of oil increases, a firm replaces an oil-fired plant with a gas-fired plant.
b. A bookseller reduces its sales staff by 60 percent after it sets up an Internet outlet.
c. Over the period $1970-2000,$ a typesetting firm decreases its employment of typesetters by 200 workers and increases its employment of computer operators by 100 workers.
d. After a successful unionization drive for clerical workers, a college buys personal computers for its faculty and reduces its secretarial workforce.