Calculating attribute closures:
{A}+ = {A}
{B}+ = {A, B, C, D} <--- Minimum candidate key
{C}+ = {A, C, D}
{D}+ = {D}
{A, B}+ = {A, B, C, D} <--- Superkey
{A, C}+ = {A, C, D}
{B, C}+ = {A, B, C, D} <--- Superkey
{A, D}+ = {A, D}
{B, D}+ = {A, B, C, D} <--- Superkey
{C, D}+ = {A, C, D}
{A, B, C}+ = {A, B, C, D} <--- Superkey
{A, B, D}+ = {A, B, C, D} <--- Superkey
{A, C, D}+ = {A, C, D}
{B, C, D}+ = {A, B, C, D} <--- Superkey
{A, B, C, D}+ = {A, B, C, D} <--- Superkey