Why is it difficult to characterize the reign of Napoleon?

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Psychologists often talk of the magical number "7" - so it is generally assumed that we can keep 7 (more or less, so 5-9 is the correct answer) items in our short term memory - we know this as a result of many experiments.

we can increase this by chunking, so for example "7 8 9 5 3" is harder to remember than "78 95 3" - try it yourself!

the answer is a)