单调谐谐振回路具有如下作用（）
A. 放大
B. 频率选择
C. 调制
D. 解调
晶体管的高频参数中，有截止频率、特征频率和最高振荡频率，这三者的大小关系，（）最大，（）次之，（）最小
According to the video, portaging is:
A. a track or route
B. carrying your canoe
C. a person who carries bags at the airport
D. the age when you can port
According to the video, a fire is important for:
A. scaring away animals
B. signaling help
C. warmth and cooking
D. to show your skill
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