The slope of the production-possibilities frontier is called the marginal: product. rate of substitution. rate of transformation. cost. Choose the most correct answer by clicking a button, or by typing a letter from the keyboard and pressing the tab key. If people worked longer hours, output would increase. Producing this extra output would be economically efficient: because more output is better. only if the use of energy was not increased. only if the extra output were distributed fairly. only if the value of the extra output was greater then the value of the lost leisure. Choose the most correct answer by clicking a button, or by typing a letter from the keyboard and pressing the tab key.