C. The way the sample was chosen may overrepresent or underrepresent students taking certain language classes.
The samples he chose may not be a representative sample because the number of students per foreign language class may not be the same. Since classes have different numbers of students, one may have a very large number of students while another may have only a few. Taking equal number of students per class is not a representative sample because it doesn't represent the students correctly.
The answer is 18+(-20) because if if you were going to solve that equation you would use the absolute value of -20 and then the opposite of the sign giving you 18-20. Hope this helps:)
<em><u>ANSWER</u></em>
My answer is in the photo above
