如何将氢气引入压缩机系统？

水泥和砂、石的比例称为（）.

A.配合比

B.水灰比

C.重量比

D.质量比

—You had better _____ food to the tigers.

—Sorry, I  _____ do it again.[ ]

A. throw; will,                          

B. don't throw; won't

C. not throw; won't                      

D. not to throw; will

进行期银屑病的特点是

A．发热明显

B．同形反应

C．皮损呈钱币状

D．点状出血现象

E．渗出明显

封闭楼梯间的墙体采用耐火极限不低于（）的不燃烧体。

A.1.00h

B.2.00h

C.3.00h

D.4.00h