电磁除铁器所建立的磁场必须是什么磁场？
参考答案：
电磁除铁器所建立的磁场必须是非匀强磁场。
You look very tired today. You should stay _____ and have a good rest.
[ ]
A. in
B. in bed
C. in a bed
D. in the bed
节点
