$10)$ Two stations $($A$)$ and $($B$)$ use selective$-$reject ARQ to exchange

frames of $100$ bits at a data rate of $10$ Mbps$.$ The two stations are

connected by a full duplex line $10$ km long. Assume the velocity is $108$

m$/$s and a BER$=5$x$10-4$

$.$

The window size to achieve an efficiency of $80\%$ is:

$($a$)18($b$)17($c$)17\mathrm{.}68($d$)17\mathrm{.}684$