It is a fact that examiners who set exam questions don't sometimes know as much about the subject they are examining than some candidates sitting the paper. Such expert candidates have to bear this in mind and become familiar with the answer that the examiner expects to be given despite the expert candidate knowing that this is not necessarily the correct one! Such expert candidates need to study textbooks written at the appropriate level and sample questions to establish what they are "supposed" to know about their specialist subject.
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