past perfect - section all tenses

[Luca Nicola 0]

Hello
How are you all today?

Just a qiuck question. Where's the mistake?

By the time we arrived, the performance had already started (start).

best regards,
LB

[Minoo 344]

Hello Luca,

Your answer is correct.

The only thing I can think of is that 'already' was not in the original sentence. Although it's perfectly correct, the software won't admit it if it's additional.