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It is said that in the indirect speech the tense in the adverbial clause introduced by "since" is unnecessary to be changed. eg.

He said , "I have studied English since I was a child . "→He said that he had studied English since he was a child.

But what about the following question:

They asked me to have a drink with them. I said that it was at least ten years since I ___ a good drink.

A. had enjoyed B.enjoyed

The anser is "A". What about "B"?
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For tests/exams be strict, use the past perfect, because it defines the right tense sequence, even with since. One can dispense with it in more casual circumstances.

Direct: I said, "It's at least ten years since I (last) enjoyed a good drink."
Reported: I said it was at least ten years since I had (last) enjoyed a good drink.
Comments  
Students: We have free audio pronunciation exercises.
For me, the past perfect always, in such contexts.
Another, less frequent, but possible, IMO, use of past perfect in this reported speech case:

Reported: I said it had been at least ten years since I (last) enjoyed a good drink.

Model:
David Copperfield by Dickens, Charles - Chapter 59
I had seen nothing like this since I went away, and it quite dashed my hopes for my friend. ...

Comments?

This is a case of using the past perfect between two separate moments in the past.