QUESTION $22$

Consider the figure below, which plots the evolution of TCP$\text{'}$s congestion window at the beginning of each time unit $($where the unit of time is equal to the RTT$).$ In the abstract model for this problem, TCP sends a "flight" of packets of size cwnd at the beginning of each time unit. The result of sending that flight of packets is that either $($i$)$ all packets are ACKed at the end of the time unit, $($ii$)$ there is a timeout for the first packet, or $($iii$)$ there is a triple duplicate ACK for the first packet.

At the end of which units of time does TCP detect a triple$-$duplicate$-$ACK?