Use Relational Algebra to express the following queries on the schema FLIGHTS (DepartureCity, ArrivalCity). A pair of cities a, b is in an instance of relation FLIGHTS if there is a direct flight from city a to city b. The relation is always antireflexive: that is, there are no flights from a city to itself.

a) Cities with flights to all cities from which there is exactly one flight.


b) Cities with flights to...