$3\mathrm{.}7-1$b$.$ TCP congestion control example $($b$).$ Consider again the figure above $($in question $3\mathrm{.}7-1$a$),$ where a TCP sender sends $8$ TCP segments at t $=1,2,3,4,5,6,7,8$ and the segment sent at t$=4$ is lost, as is the ACK segment sent at t$=7.$

What is the sender action at t$=13$ upon receipt of the ACK?