Please help! Charles wants to find out if the students in foreign language classes spend more time in class speaking in English or in the foreign language they are studying. Charles first gets class lists of all students taking foreign language classes. He then chooses 10 students from each different language class to survey. Which best explains why the sample he chose may not be a representative sample?

The sample size is not large enough to represent the population.
The sample was not chosen at random.
The way the sample was chosen may overrepresent or underrepresent students taking certain language classes.
The way the sample was chosen does not sample students from each different language class.