15. 考量關聯(relation) R = {A, B, C, D, E, F, G, H, I, J} 具有功能相依性 FD = { {A, B}→{C}, {B, D} → {E, F}, {A, D} →{G, H}, {A} → {I}, {H} → {J} }，則 R 之 key 為何?
(A) {A, B, C}
(B) {A, B, D}
(C) {A, B, E}
(D) {A, B, F}