Both B and C should be true.
Obviously applying select operation before the join operation is better than plan2 bcoz it helps in optimising the entire Join-Selection process.
A is false.
B is true.
suppose two students paid more than x opting for same course so both will be selected as per select operation in plan1 and then we will apply Join between result of select and enrolled printing same course twice.
C is true.
Effect of select operation becomes more evident when x is any value >=6000(min. amount) bcoz then select will remove these tuples at very first place instead of removing them after join.
But even if x=5000 then select operation though becomes useless bcoz all amounts are greater than or equal to 6000.But still Plan1 is faster than Plan2 bcoz it has to apply select(scanning the tuples) on less number of tuples as compared to Plan2 which will apply select after JOIN leading to scanning of more tuples(although none would be eliminated).
D is false
D would have been true ---> "For x = 9000, Plan I executes faster than Plan 2 for all databases"