The ________ they heard the alarm, they fell in for action.
A. immediate B. instantly C. instant D. momently
The ________ they heard the alarm, they fell in for action.
A. immediate B. instantly C. instant D. momently
C
【解析】句意：他们一听到警报，就立即集合准备战斗。the instant 表示“一……就……”，可以直接连接从句。