Which of the following relational calculus expressions is not safe ?
- $\left\{t \mid \exists u \in R_1\left(t[A] = u[A]\right) \land \neg \exists s \in R_2 \left(t[A] = s[A]\right)\right\}$
- $\left\{ t \mid \forall u \in R_1\left(u[A]= "x" \Rightarrow \exists s \in R_2\left(t[A] = s[A] \land s[A] = u[A]\right)\right) \right\} $
- $\left\{t \mid \neg (t \in R_1)\right\} $
- $\left\{t \mid \exists u \in R_1\left(t[A]=u[A]\right) \land \exists s \in R_2 \left(t[A] = s[A]\right)\right\}$
- (i)
- (ii)
- (iii)
- (iv)
