The correct functional group $X$ and the reagent/reaction conditions $Y$ in the following scheme are:
$X-(CH_2)_4-X \xrightarrow{Y} \text{condensation polymer}$
$(i) \text{ with } HOOC-(CH_2)_4-COOH / \text{heat}$
$(A) X = COOCH_3, Y = H_2 / Ni / \text{heat}$
$(B) X = CONH_2, Y = H_2 / Ni / \text{heat}$
$(C) X = CONH_2, Y = Br_2 / NaOH$
$(D) X = CN, Y = H_2 / Ni / \text{heat}$

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