System A needs to send a $10$ Mb file to System B using a Sliding Window flow control with a window size $($W$)$ of $7.$ Both Systems A and B have buffers large enough to handle the sliding window protocol. Each packet will contain L $=1024$ bits of data, the distance between A and B is d $=2$ km$,$ the propagation velocity is V $=2$ x $108$ m$/$sec$,$ and the transmit data rate is N $=500$ Mbps$.$ What is the utilization rate of this transmission?

a$.65\%$

b$.100\%$

c$.20\%$

d$.91\%$