Which is the correct answer and why?
Let us go to the carnival, (shall we / shan't we / should we / shouldn't we)?
All but Stella (is being / were being / has been / have been)auditioned for the ballet performance this afternoon.
In a year's time, he (is becoming / had become / shall become / will have become) a qualified doctor.
In my opinion, the answer for Q1 is 'shall we'. However, in question tags, the question tag is negative if the main part of the sentence is positive. It doesn't seem to apply here so I'm not sure.
As for Q2, 'All is the subject' but I've no idea what the answer is.
In Q3, I think the answer is 'shall become' which describes the future. 'Is becoming' is unlikely to be the answer as it means 'gradually changing'. I think the future perfect tense 'will have become' is also an answer as it describes a future action that will have been completed.