Question

The Chinese Han dynasty expanded its rice fields. Both places had used/were using/had been using/used metallurgical techniques to produce weapons.

Please choose the correct tense.

Answer 1

Since this is a grammar question, I would choose a simple past.

Used would be my choice.

But you need some sort of context to get it right. had used implies that they no longer do.

Were using means that they used techniques that are no longer in use.

had been using has the same sort of meaning as the previous one. They used it in the past and it was in constant use then.

You really do need the context for this question. I would choose used, but I don't want to guarantee it. Any one of these could be correct, depending on what you were talking about.