Option C
$1 minute=4000$ $revolution$
$60 sec=4000$ $revolution$
$1 sec=4000/60$ $revolution$
$or$ $we$ $can$ $say$ $1$ $revolution=60/4000 sec=15 millisecond$
$average$ $ access$ $time$ $for$ $a$ $Drum$ = $time$ $in$ $half$ $revolution $
$=15/2$
$= 7.5millisecond$