The production department is proposing the purchase of an automatic insertion machine. it has identified 3 machines, each with an estimated life of 10 years. which machine offers the best internal rate of return? machine a machine b machine c annual net cash flows $50,000 $40,000 $75,000 average investment $250,000 $300,000 $500,000 a. machine b only b. machine c only c. machine a only d. machines a and b