协调性子宫收缩的特点是

A．宫口扩张和先露下降不呈比例

B．子宫收缩仍保持正常的节律性，对称性和极性

C．只保持正常的节律性和对称性，极性倒置

D．宫缩频繁，但宫口不开大

E．宫缩的持续时间长，间歇时间短

As a student, you should do your duty and put your heart into study ______.

[      ]

A. at one time

B. at all times

C. at a time

D. at times