hard at work 一生懸命働く

hard at work = hard at it は一生懸命せっせと働くことです。6:25 the system that seems to be broken was actually hard at work. 麻痺しているように見えるカナダのシステムも実は着実に機能している。

RMR: Canada Explained
https://www.youtube.com/watch?v=A0XUQa55OhI  1:32 throughout the night, tens of thousands of Filipinos are hard at work. フィリッピンでは職が掛け替えのないものだから、
BBC HardTalk On The Road in the Philippines
https://www.youtube.com/watch?v=SaPGyraw0nc  0:29 Department of Transportation road crews are hard at work today. 運輸省の道路局職員たちは道路の凸凹の復旧に余念がない。

練習問題解答
a-5411 hard at work
b-4201 hard at work