A chemistry professor who teaches a large lecture class worries about being boring. So she sends out surveys to the stud

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A chemistry professor who teaches a large lecture class worries
about being boring. So she sends out surveys to the students who
are NOT coming to class regularly and asks about how she can make
the class more interesting, hoping she can get more students than
the 50% who usually attend. This survey method violates which
caution about sample survey the most? Explain your answer in
detail. (At least 3 sentences are needed) (More than one
answer may be correct, it depends on your defense of your
answer)