Answer:
The correct answer is option a.
Explanation:
Opportunity cost can be defined as the cost of giving up or sacrificing the second-best alternative. In other words, it is the benefit that could have been earned if the alternative was not sacrificed.
Here, Joe bought gold coins for $1,000 and sold them at the same price. He could have instead invested in a certificate of deposit and earned a 3% interest rate. So here the opportunity cost of purchasing the gold coins is the interest earnings sacrificed.
I assume you mean in the past, and not the near past, but something like a century ago. Salt was a precious commodity used for preserving meats longer as people couldn't freeze it; plus, it added flavor to dull, unseasoned food. Iron is a strong alloy used in lots of things, and a century ago, people loved METAL machines that could get work done faster.
Question is not clear enough.