6. Place the numbers $1,2, \cdots, 8$ at the eight equally spaced points on a circle such that the sum of any two adjacent numbers is a prime number. If rotations of the circle that result in the same arrangement are considered identical, then the number of different arrangements is ( ).
(A) 4
(B) 8
(C) 12
(D) 16
6. A.

If the sum of two adjacent numbers is an odd prime, then the numbers on the circle alternate between odd and even.

Thus, the numbers adjacent to 8 must be 3, 5, 7, and the numbers adjacent to 3, 5, 7 must be 4, 6. The remaining two numbers, 1 and 2, must be adjacent to each other, and 1 must be adjacent to either 4 or 6.

Considering the adjacency of the three modules [4,7,6], [5,8,3], [1,2], we get 4 possible arrangements.