Tony : If the driver in front hadn’t stopped so suddenly.

I would have been able to avoid the accident.

Albert : Well, it’s over. Let me take you to the hospital. You are seriously hurt.
From the underlined expresses we know that Tony …
True answer:
could not avoid the accident

The underlined sentence is a type III conditional which means it must contradict the facts in the past tense.

  The Structure of the Descriptive Text