In my country, this zither is usually in D key. What key should it be if it is played with erhu and bamboo flute?

folk music ensemble: if you add guzheng, it is usually in D or G key. I played the pipa in the key of D, but we played it in the key of G in the ensemble.

I suggest you try both, because different instruments and different arrangements will have different feelings. Practice is the most important thing.

if you don't understand anything, you can ask me. Thank you~