Student or Learner
The following sentence comes from one of China's college entrance examination papers (2006):
Father ____ for London on business upon my arrival, so I didn't see him.
A. has left B. left C. was leaving D. had left (The given answer is D.)
I'm confused about this sentence. Since upon means immediately after, how can we say "Father had left for London on business upon my arrival"? If we say "Father left for London on business upon my arrival", this sentence makes sense, but it doesn't match "so I didn't see him".
Could you please, sir, explain the sentence?
"On my arrival [home/at the office, I found that] Father had left..."
There is an implied sense that the person arrives somewhere after the other person had already departed.