循行于后正中线的经脉是（）
A.冲脉
B.督脉
C.任脉
D.带脉
E.胆经
参考答案：B
There was no ______ that she was a good teacher.
A．reason
B．fact
C．doubt
D．wonder
应采取措施消除过量氟的浓度是（）
A．＜0.3ppm
B．0.4～0.6ppm
C．0.7～1.0ppm
D．1.5ppm
E．2～4ppm
