The relational algebra expression equivalent to the following tuple calculus expression:
$\left\{t \mid t \in r \land \left(t[A] = 10 \land t[B]=20\right)\right\}$ is

$\sigma_{(A=10 \lor B=20)} (r)$

$\sigma_{(A=10)} (r) \cup \sigma_{(B=20)} (r)$

$\sigma_{(A=10)} (r) \cap \sigma_{(B=20)} (r)$

$\sigma_{(A=10)} (r)  \sigma_{(B=20)} (r)$