胸部创伤时，X线平片上不能显示的是（）
A.开放性气胸
B.血胸
C.纵隔及皮下气肿
D.肋软骨骨折
E.张力性气胸
参考答案：D
Feeding birds is ________ than others. [ ]
A. an interesting work
B. a very interesting job
C. a more interesting job
D. a much more interesting work
乳化液泵电动机两端轴承一般要加（）。
A.氧气
B.水油
C.水
D.高温润滑脂
