The answers to the sub-questions are:
(a) \( (B \cap C)^{\prime} = \{1, 3, 5, 6, 7\} \)
(b) \( B^{\prime} \cup C = \{1, 4, 5\} \)
Thus, the final answers are:
\[
\boxed{(B \cap C)^{\prime} = \{1, 3, 5, 6, 7\}}
\]
\[
\boxed{B^{\prime} \cup C = \{1, 4, 5\}}
\]