7. Find a complete residue system $r_{1}, \cdots, r_{4}$ modulo 4 and a complete residue system $s_{1}, \cdots, s_{5}$ modulo 5, such that $\square$
(i) $r_{i} s_{j}(1 \leqslant i \leqslant 4,1 \leqslant j \leqslant 5)$ is a complete residue system modulo 20;
(ii) $r_{i}+s_{j}(1 \leqslant i \leqslant 4,1 \leqslant j \leqslant 5)$ and $r_{i} s_{j}(1 \leqslant i \leqslant 4,1 \leqslant j \leqslant 5)$ are simultaneously complete residue systems modulo 20.