Consider the following relation and associated functional dependencies

$R(A,B,C,D)$

$ABC$

$CD$

$DA$

What are the keys of $"$R$"?$ hint, you will have to take the closure of all combinations of $A,B,C,$ and $D).$

$\{A\}+=$

$\{B\}+=$

$\{C\}+=$

$\{D\}+=$

A

B

CD

$AB$

$BC$

$BD$

$AC$

$\{AB\}+=$

$\{AC\}+=$

$\{AD\}+=$

$\{BC\}+=$

$\{BD\}+=$

$\{CD\}+=$

$\{ABC\}+=$

$\{ABD\}+=$

$\{ACD\}+=$

$\{BCD\}+=$

A$.$

$3$

CD

$AB$

$BC$

$BD$