宫口未开全，下列哪项会使产妇过早的屏气()
A．枕前位
B．枕后位
C．臀位
D．横位
E．足位
参考答案：B
I couldn't sing English songs _____.
A. for three years
B. in three years
C. three years ago
D. three years before
群体免疫力(herd immunity)
