What is $$ X if X$=\{0,1,2,3,7,8\}?$

a$.$

$\{1,2,3,4,5,6,7,8\}$

b$.$

$\{1,1,2,3,4,5,6,7,7,8\}$

c$.$

Both of the above are valid solutions.

d$.$

None of the above.