The quick brief guide to differentiate 'true, false and not stated' statement :
Both true and false statement require evidence form the text (from either one or both linear and non-linear text). The evidence could also be inferred.
True statement has an evidence that supports it / agrees with it.
False statement has an evidence that goes against it / disagrees with it.
Not stated is for statement that has no contextual evidence which agrees or disagrees with it, even if you infer.
You should not use your existing knowledge when responding to questions in 800/3.

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