Can someone explain it to me as to why in selective repeat it's not possible for an acknowledgment to come for a packet that falls outside the current window? Because it may be possible that there is some delayed acknowledgment. It's possible in all sliding window protocols then why is statement 2 only true?
Moreover, in the solution, they mentioned that statement 2 is true because GBN has cumulative ack because of which if we receive ack 2 then the sender will assume that both packets 1 and 2 have been received successfully and so it slides the window to remove 1 and 2 from it but later we might get ack 1 which I feel is not possible because here we are talking about cumulative ack not independent. So how is this reason valid?