B , Because , when First pkd sent with ACK no. 1, receiver received value= 1+1=2, and then Sender Send the next pkt with ACK 2 ,but , if the time period was over before reaching the pks of Ack 2, receiver the the previous Ack 1 +1 to Sender, and the sender again send the pkt of ACK 2, And this process repeat ...