What is the mathematical expectation of choosing five out of seven in college entrance examination English?

The mathematical expectation of the number of correct answers is 5/7. In addition, popular science:

Mathematical expectation: random guessing (for example, if you choose A F D E B) is the same as guessing all the answers (A A A A A A A A A A A A A A A A A A A A A A), which are all 5/7.

Variance: The variance of random guess is greater than the variance of all answers. Quite simply, you may have guessed all the answers right, but all the guesses can only be right at most.

If you want to seek wealth and risk, please guess five different answers directly. To be stable, please guess five identical answers directly.

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