It is my understanding that during the closing auction, the selected closing price is the price at which the most number of shares would trade. But what happens if there is more than one such price? Suppose person X places a limit on close (LOC) order to buy 100 shares at $11, person Y places a LOC order to sell 100 shares at $9, and X and Y are the only participants in the closing auction. All prices $9 <= p <= $11 are valid closing prices. How would the closing price be chosen in this case?
(If the answer depends on jurisdiction, I would like an answer for the NYSE, Nasdaq, Hong Kong Exchange and/or the London Stock Exchange.)