Discussion
In a railway reservation system, the entities are-date, train no, place of departure, destination, type of train, type of seats, seats available.
The primary key will be :
Answer: A
The primary key is train no. + date.
A primary key must identify each reservation record uniquely. A given train number runs every day (or on several days), so the train number alone repeats; the combination of train number and date picks out one particular journey, on which seat availability, seat type and the rest depend.
Train number plus destination, or plus place of departure, still repeats from day to day, since the same train serves the same stations daily, and the train number alone is not unique either.
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