18. There are three types of people, A, B, and C, totaling 25 people. A type people always tell the truth, B type people always lie, and C type people alternate between telling the truth and lying (for example, if a C type person tells the truth this time, the next thing they say will definitely be a lie, and the one after that will be the truth again).
The priest asked everyone, “Are you an A type person?” 17 people answered “yes”.
The priest then asked everyone, “Are you a C type person?” 12 people answered “yes”.
The priest then asked everyone, “Are you a B type person?” 8 people answered “yes”.

Among these 25 people, ( ) are C type people.
【Analysis】Let the number of people in class $\mathrm{A}$ be $\mathrm{A}$, the number of people in class $\mathrm{B}$ be $\mathrm{B}$, the number of people in class $\mathrm{C}$ who tell the truth $\rightarrow$ lie $\rightarrow$ tell the truth be $\mathrm{C}_{1}$, and the number of people in class $\mathrm{C}$ who lie $\rightarrow$ tell the truth be $\mathrm{C}_{2}$. Then, 
\[
\left\{\begin{array}{c}
\text { The first question answered "yes" by: } \mathrm{A}, \mathrm{~B} \text { and } \mathrm{C}_{2} \\
\text { The second question answered "yes" by: } \mathrm{B} \text { and } \mathrm{C}_{2} \\
\text { The third question answered "yes" by: } \mathrm{C}_{2}
\end{array}\right.
\]
So, 
\[
\left\{\begin{array}{c}
\mathrm{A}+\mathrm{B}+\mathrm{C}_{2}=17 \\
\mathrm{~B}+\mathrm{C}_{2}=12 \\
\mathrm{C}_{2}=8
\end{array}\right.
\]
Solving, we get: 
\[
\left\{\begin{array}{l}
A=5 \\
B=4 \\
C_{2}=8
\end{array}\right.
\]
Therefore, the number of people in class $\mathrm{C}$ is $25-5-4=16$ people.