7) But of course you know all about that - you must - you have been a doctor for over thirty years! That's the reason I want to talk to you. You 8) work as a GP for your whole career, and I want to ask you - 9) specialise? And when 10) (you/ever regret) your decision not to (you/know) that this was the kind of doctor you wanted to be? I 11) (start) thinking about what to do after I finish my training, you see.
Чтобы правильно вставить слово, необходимо знать грамматическую структуру английского предложения и понимать смысл текста.
Ответ: must, have been