Answer:
The correct answer is: had been towed
Explanation:
In this sentence, we should use past perfect tense to indicate an action that happened before some other action in the past.
The past perfect is formed with the past tense form of the verb to <em>have </em>(had) and the past participle of the verb.
In the example above, we can notice that the wrecked vehicles had been towed before we arrived on the scene.
So, the first action will always contain past perfect tense, while the second one will always contain past simple tense.
