Answer: . two-stage area
Explanation:
In two-stage specimen sampling, a simple random sample of specimen is selected and then a simple random sample is selected from the units in each sampled specimen. Two-stage sampling is used when the sizes of the specimens are large, making it difficult or expensive to observe all the units inside them.
The opportunity cost of attending class is the $15 that could have been made by watching a neighbor's child.
Opportunity cost refers to the benefits that one gives up in order to enjoy another benefit, that is, the benefit that is sacrificed.
In this question, two benefits are given up, but the real opportunity cost is the one that have the highest value, which is the $15.
