What are the definitions of opportunity cost, scale of preference, and how to calculate the median?

Opportunity cost is the value of the next best alternative foregone. It is often referred to as the real cost of a decision.
The correct option is (d).
Answer for 1: $\boxed{\text{real}}$

The order of ranking needs according to their importance is known as a scale of preference. This helps individuals and societies make choices when faced with scarcity.
The correct option is (c).
Answer for 2: $\boxed{\text{scale of preference}}$

To find the median, first arrange the scores in ascending order: $2, 5, 15, 18,