Assume that for a TCP sender$,$ the congestion window size $cwnd=4,$ the slow start threshold ssthresh $=20,$ and there is no time out or triple duplicate ACKs. What will be the congestion windows size in the next transmission round, i$.$e$.,$ after one RTT$?($Hint: Since cwnd $$ ssthresh, the sender is in the slow$-$start phase.$)$

$15$

$25$

$8$