Let $R_1 \left(\underline{A}, B, C\right)$ and $R_2\left(\underline{D}, E \right) $ be two relation schema, where the primary keys are shown underlined, and let C be a foreign key in $R_1$ referring to $R_2$. Suppose there is no violation of the above referential integrity constraint in the corresponding relation instances $r_1$ and $r_2$. Which of the following relational algebra expressions would necessarily produce an empty relation?