a) Consider the design of a communication system.

i. How many three-digit phone prefixes that are used to represent a particular geographic area (such as an area code) can be created from the digits 0 through 9?

ii. As in part i., how many three-digit phone prefixes are possible that do not start with 0 or 1, but contain 0 or 1 as the middle digit?

iii. How many three-digit phone prefixes are possible in which no digit appears more than once in each prefix?