When users log in to banking systems from un$-$ familiar devices, banks often send them a text message or email containing a $6$ digit verification code. These verification codes typically only use the chosen from $0,1,2,3,4,5,6,7,8,9.$

$($a$)$ How many different verification codes are possible?

$($b$)$ How many verification codes do not contain a repeated digit?

$($c$)$ How many verification codes contain one or more re$-$ peated digits?