P$22.$ Consider the GBN protocol with a sender window size of $4$ and a sequence number range of $1,024.$ Suppose that at time t$,$ the next in$-$order packet that the receiver is expecting has asequence number of k$.$ Assume that the medium does not reorder messages. Answer the following questions:

a$.$ What are the possible sets of sequence numbers inside the senders window at time t$?$ Justify your answer.

b$.$ What are all possible values of the ACK field in all possible messages currently propagating back to the sender at time t$?$ Justify your answer.