$$ \begin{array}{|c|c|c|} \hline a & \text{Repeater} & 1 & \text{physical layer} \\ \hline b & \text{Router} & 2 & \text{data link} \\ \hline c & \text{Gateway} & 3 & \text{all layer} \\ \hline d & \text{Bridge} & 4 & \text{network layer} \\ \hline \end{array}$$
- a-1,b-4, c-3,d-2
- b-1,a-4, c-3,d-2
- a-1,c-4, b-3,d-2
- d-1,b-4, a-3,c-2