To store text characters in binary, a computer uses an encoding scheme such as ASCII

or Unicode. The idea is to encode characters as certain sequences of bits $$ A might be

represented by $0001,$ B might be represented by $0010,$ and so on$.$ When the computer sees a

particular sequence of bits, it interprets that sequence as its character equivalent.

The number of bits used in an encoding scheme determines how many unique characters can be

represented. For example, consider three encoding schemes: scheme S$1$ uses $1$ bit per character,

scheme S$2$ uses $2$ bits per character, and scheme S$3$ uses $3$ bits per character. Because each

individual bit has only two possible values $(0$ or $1),$ S$1$ can represent up to two unique characters

$($encoded as $0$ or $1).$ S$2$ can represent up to four unique characters $($encoded as $00,01,10,$ or

$11),$ and S$3$ can represent up to eight unique characters $($encoded as $000,001,010,011,100,$

$101,110,$ or $111).$

$($a$)(2$ points$)$ In an encoding scheme that uses $4$ bits per character, what are the possible

sequences of bits? There should be $16$ in all.

$1$