Most of the choices appear to be             ones that you are familier with, but after taking              second

look, you will find that they are different.[ ]

A. /; the

B. the; the

C. the; a     

D. /; a

钢结构中管、球加工中，半球圆形坯料钢板应用（）或等离子切割下料。

A.乙炔

B.氧乙炔

C.甲醛

D.丙炔