It couldn’t be row A or B because there would then be a possibility the ball was in A6 or B5 and that Bernard actually did know where the ball was. Albert says he knows Bernard doesn’t know where the ball is, which means he is 100% confident, and the only rows that allow for that confidence are rows C and D, because each ball in each row is not alone in its column.
Scenario 1: The madman tells you the ball is in row A. Then the madman whispers the column to Bernard. Can you really be sure that Bernard knows or doesn’t know where the ball is?
Scenario 2: The madman tells you the ball is in row C. From this, you can be completely sure that Bernard doesn’t know where the ball is, because any column he would be given doesn’t narrow it to a single row.
It couldn’t be row A or B because there would then be a possibility the ball was in A6 or B5 and that Bernard actually did know where the ball was. Albert says he knows Bernard doesn’t know where the ball is, which means he is 100% confident, and the only rows that allow for that confidence are rows C and D, because each ball in each row is not alone in its column.
Scenario 1: The madman tells you the ball is in row A. Then the madman whispers the column to Bernard. Can you really be sure that Bernard knows or doesn’t know where the ball is?
Scenario 2: The madman tells you the ball is in row C. From this, you can be completely sure that Bernard doesn’t know where the ball is, because any column he would be given doesn’t narrow it to a single row.