Suppose T₁(P, Q) and T₂(R, S) are two relation schemas. Let t₁ and t₂ be the corresponding relation instances. R is a foreign key that refers to P inT₁. If data in t₁ and t₂ satisfy referential integrity constraints, which of the following is/are always true?

I. ∏_R (t₂) – ∏_P(t₁) = Φ

II. ∏_P(t₁) ≠ ∏_R(t₂)

III. ∏_P (t₁) – ∏_R (t₂) = Φ