患者，女，42岁左下6，缺失。缺牙区牙槽嵴吸收严重，且不平整，左下75基牙条件尚可，医师设计全自洁型桥体式金属全冠桥修复。

全自洁桥体多用于 （）

A.上颌前牙

B.下颌前牙

C.上颌后牙

D.下颌后牙

E.上颌前后牙均有缺失时

  I was really anxious about you. You _______ have left home without a word.

A．mustn't

B．shouldn't

C．needn't

D．couldn't