Find the set $A \cup U$. \[ \begin{array}{l} U=\{1,2,3,4,5,6,7,8\} \\ A=\{5,6,7,8\} \end{array} \] Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. $A \cup U=\{\}$ (Use a comma to separate answers as needed.) B. $A \cup \cup$ is the empty set.

Step 1 ：First, understand the meaning of the union of sets. The union of two sets A and U, denoted by A ∪ U, is the set of elements which are in A, in U, or in both A and U.

Step 2 ：In this case, set A is {5,6,7,8} and set U is {1,2,3,4,5,6,7,8}.

Step 3 ：The union of A and U includes all the elements that are in A, in U, or in both.

Step 4 ：Since all elements of A are also in U, the union of A and U is just U.

Step 5 ：So, A ∪ U = {1,2,3,4,5,6,7,8}.

Step 6 ：Check the result: all elements of A and U are included in the union, so the answer is correct.

Step 7 ：\(\boxed{A \cup U = \{1,2,3,4,5,6,7,8\}}\)