Do all the sentences below sound fine to you? The original one is:
The phone ______ (ring) as I ______ (leave) the house.
That's what I came up with:
The phone was ringing as I left the house (the former considered a process, the latter considered a completed action)
The phone rang as I was leaving the house (the former viewed as a completed action, the latter - as a process)
The phone was ringing as I was leaving the house (two processes happening simultaneously)
But the problem is as follows. What if we extend the original sentence, adding "but I didn't answer it'?
The phone ____ (ring) as I ____ (leave) the house, but I didn't answer it.
The key gives "the phone rang as I was leaving the house, but I didn't answer it" as the correct alternative, implying that the process of leaving was interrupted by the call.
I do totally agree with the key, but I cannot explain why the other two alternatives are incorrect. Does it have anything to do with 'but I didn't answer it'?
The addition of 'but I didn't answer it' changes nothing in this sentence. All three versions are still possible.