source :- Forouzan
>>>when there Time out occurs then TCP acts strongly :
A:- It sets the valuee of threshold to one half the current window size.
B:- it sets cwnd the size of one segment .
C: it strats slow -start phase again.
>>> when detection is by ACK's i.e. three ACK's received then :
A:- - It sets the valuee of threshold to one half the current window size.
B: sets CWND to threshold.
C: congestion avoidance phase starts.
In This question starting window size is two . That's why it must be two , four, eight ...