Assume that the u. s. ppf measures steel production on the x-axis and production of services on the y-axis. if robots are the only technological advance, a. the u. s. ppf does not change b. the u. s. ppf shifts outward, increasing the x-axis intercept and the y-axis intercept c. the u. s. ppf rotates outward, increasing the x-axis intercept with no change in the y-axis intercept d. the u. s. ppf rotates outward, increasing the y-axis intercept with no change in the x-axis intercept