Answer :

If you are asking for a pc game, then it is the key registered by the game to which it is more responsiveif more than one keys are pressed.
pmkkav5
Primary key shoud not allowed null and repeated values,student id and employee id best example for promary key.
    A table should not have more than one primary key .it should have only one primary key. 
   But combination of more than one columns can be populate one primary key.

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