這一句是用 "in" 定 "during", 為什麼？

During 除可意為從開始到結束的整個期間外
During the summer she worked as a lifeguard –Longman
亦可意為整個期間的某一特殊時點
My father was killed during the war. -- Longman
In the afternoon是最常見用法, 因為afternoon是時段而非一個時點. 但當強調afternoon是個時點時，也可用at the afternoon
如
it is 2 o’clock IN the afternoon/下午是個期間，期間中有二點的時候並不否定它是期間
AT the afternoon meeting etc /afternoon 時間被meeting(at the meeting)限定住成為一個特定時點，故用At

They stopped work for half an hour (In / During) the afternoon.
他們在下午的時間中(區間 –沒有被限定住為特定時間)停止工作半小時，故用in the afternoon。若用during不符合Longman句一(不是整個下午在睡覺)及句二(有具體提示有一個特定時點存在)之解釋

During== as a preposition which shows throughout the continuance of ; or, at some point of time in the continuance of. 在....期間; 或者在.....期間之某一時間. in==As a preposition which indicates of time when (指時間) 在.....之時; or, ( of time) in course of ; within the space of: (指時間) 過(若干時間); 在(若干時間)內. 按照以上的解釋,本人認為應該用' "in " 較為適合. Yip

They stopped to work for half an hour during this afternoon.
(✅) 他們在下午時段 停工了一個半小時​​。 (這個下午)

P.S. 如果用"in the afternoon”，是表示: "每天中午打後 停休一個半小時​​"。