More Problems with the Past Perfect

(Grammar Questions I Couldn't Answer)

This is a follow-up of sorts to the previous post.

Right about the same time I was experiencing problems with the past perfect in my own class (see previous post) a colleague approached me with his own questions about the past perfect.

His questions were based on an exercise from the textbook Life Intermediate lesson 3B Return to the Titanic p.36-37 .  The lesson contained grammar exercises on the past perfect, which included the following sentence:

This ship and several others _____________ (sink) in 1671 when they _____________ (hit) rocks.

The students were supposed to put the verbs in brackets in the correct tense.

My own native speaker intuition was telling me that the past simple would be the most natural in both cases, but that if one of them had to be in the past perfect, then it would be the second verb.

The answer key to these exercises (located in the back of the textbook and in the teacher's book) said that the correct answer was "had sunk" and "hit".

In other words, the sentence was supposed to read:

This ship and several others had sunk in 1671 when they hit rocks.  

I told my colleague that my best guess was that the publishers of the textbook had simply made a mistake.  Other than that, I didn't know how to explain the sentence.