Suppose that $\{a,b\}$ is a candidate key for a relation with schema $(a,b,c,d).$

Which of the following could be candidate keys?

$\{$c$\}$

$\{a,b,d\}$

$\{$a$\}$

$\{a,c\}$

$\{a,b,c\}$

$\{$b$\}$