A shell and tube heat exchanger is to be used in a corrosive service. If carbon steel tubes are used, the cost of the exchanger is estimated to be $160,000, and it is expected that the exchanger will need to be replaced after two years. If stainless steel tubes are used, the service life will be increased to five years, but the cost increases to $400,000. If the cost of capital is 15%, which option should be selected?