D
Because opportunity cost is a benefit profit or value that firm has to give up in order to achieve or have something else
The last sentence is an example of a comma splice since 2 independent clauses are connected by only a comma(it needs a coordinating conjunction after the comma or change it to semicolon or period)
Well, since Elaborate is a synonym for explain, I would guess that the student would just have to explain more about the topic.
Try b or d those are the most possible answers