your firm considered the replacement of an existing piece of equipment that originally cost $70,000, is presently shown on the company records with a value of $25,000 but has a present market value of only $19,000. For purposes of an engineering economic analysis of whether to replace the equipment or keep it, what is the opportunity cost if the firm decided to keep the equipment? O a. $70,000 O b. $60,000 O c. $25,000 O d. $45,000 O e. $19,000