Question
Suppose R1(A, B) and R2(C, D) are two relation schemas. Let r1 and r2 be the corresponding relation instances. B is a foreign key that refers to C in R2. If data in r1 and r2 satisfy referential integrity constraints, which of the following is ALWAYS TRUE

πB(r1)  πC(r2) = Ø

πC(r2)  πB(r1) = Ø

πB(r1) = πC(r2)
Answer is
"pB(r1)  pC(r2) = Ø"
please post an explanation
anyone have any solutions