<|im_start|>system
You are a helpful assistant. You first think about the reasoning process in the mind and then provide the user with the answer. You should try to truely solve the question by yourself without cheating.  <|im_end|>
<|im_start|>user
 A high school offers 3 elective courses, and there are 3 students, each choosing only one course. Exactly one course will have no students choosing it. There are $\boxed{\text{number}}$ different course selection schemes (answer in digits). Show your work in <think> </think> tags. And return the final answer in <answer> </answer> tags.<|im_end|>
<|im_start|>assistant
Let me solve this step by step.
<think>
user
18
A high school offers 3 elective courses, and there are 3 students, each choosing only one course. Exactly one course will have no students choosing it. There are $\boxed{\text{number}}$ different course selection schemes (answer in digits).
test