フリー問題

問題文

A dashboard that starts up must show the current state of 3000 valves immediately, but the valves only publish when their state changes and some have not changed for weeks. Which MQTT feature gives the dashboard the current state on connection?

選択肢

1. The dashboard gets current state from a retained publication per topic.
2. The dashboard gets current state from a timed republication per topic.
3. The dashboard gets current state from a replayed history per topic.
4. The dashboard gets current state from a persistent session per topic.