John's dairy makes and sells homemade ice cream. He uses a machine he bought 15 years ago for $1,500. The machine still works but is so old that nobody else would be willing to buy it, as they would prefer buying a new one that would last longer. John's opportunity cost of continuing to use this machine is

zero

Explanation:

Opportunity cost is simply defined as what an individual forgoes when he or she is confronted with choices to make and makes a particular economic decision.

In this scenario, since we are told that nobody else would be willing to buy the machine, as they would prefer buying a new one that would last longer. It implies that John's opportunity cost of continuing to use this machine is zero since he has nothing to lose.

c.

clarify customers’ doubts only when asked.

the answer to the question :

which best describes a central bank's primary goals? limiting inflation and reducing unemployment reducing unemployment and maintaining cash flow controlling stagflation and reducing unemployment managing credit and ensuring the money supply's liquidity

is:

managing credit and ensuring the money supply's liquidity