In stop$-$and$-$wait method of flow control, if $1000$ good data frames are sent and received how many ACK frames have been sent by the receiver?

$($a$)50$

$($b$)99$

$($c$)1000$

$($d$)$ neither a nor b nor c

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A sender has a sliding window of size $1000.$ The first $200$ frames are sent. The first ACK received is ACK $20.$ What frame is the receiver expecting?

$($a$)19$

$($b$)20$

$($c$)201$

$($d$)200$

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