8. A number like 45132 is called a "wave number", meaning the tens and thousands digits are larger than the digits adjacent to them. The number of five-digit wave numbers that can be formed using the digits $1,2,3,4,5$ without repetition is ( ).
(A) 20
(B) 18
(C) 16
(D) 11
8. C.

According to the problem, we can divide it into two cases.
(1) The thousand's and ten's place are 5 and 4, there are $\mathrm{A}_{2}^{2} \mathrm{~A}_{3}^{3}$ ways;
(2) The thousand's and ten's place are 5 and 3, there are $\mathrm{A}_{2}^{2} \mathrm{~A}_{2}^{2}$ ways.

Therefore, there are a total of $12+4=16$ ways.