什么是食管癌腔内型?

素质教育就是不要考试，特别是不要百分制考试。这句话对吗?为什么?

硫化钴

已知一脉冲信号其周期为1us，经过一个10分频器后，输出脉冲的周期为（）。

A.0.1us

B.10us

C.20us

D.0.05us

A 15-year-old boy is admitted to the health care facility after acting out his aggressions inappropriately at school. Predisposing factors to the expression of aggression include which of the following

A．Violence on television.

B．Passive parents.

C．An internal locus of control.

D．A single-parent family.

一般情况下，发生瓦斯爆炸的上限浓度为（）

A.17%

B.16%

C.15%