Define sets U, A, B, and C as shown below. Find (AUBUC)'.
\[
U=\{a, b, c, d, e, f, g, h\} \quad A=\{d, g, h\} \quad B=\{a, g, h\} \quad C=\{a, b, c, e, f\}
\]
Select the correct choice below and, if necessary, fill in the answer box within your choice.
A. $(A \cup B \cup C)^{\prime}=$
(Use a comma to separate answers as needed. Type each answer only once.)
B. $(A \cup B \cup C)^{\prime}=\varnothing$

Step 1 ：Define the sets U, A, B, and C as given in the problem.

Step 2 ：Find the union of sets A, B, and C. This includes all elements that are in A, B, or C.

Step 3 ：Find the complement of the union of sets A, B, and C. This includes all elements in the universal set U that are not in the union of A, B, and C.

Step 4 ：After calculating, we find that the union of sets A, B, and C includes all elements in the universal set U.

Step 5 ：Therefore, the complement of the union of sets A, B, and C is an empty set, which means there are no elements in U that are not in the union of A, B, and C.

Step 6 ：Final Answer: \((A \cup B \cup C)^{\prime}=\boxed{\varnothing}\)