are the two students doing in the classroom? [ ]
A. Who
B. What
C. Where
D. Whose
答案:B
一钢管直径d=1.0m,厚度δ=1.0cm,钢材允许应力[σ]=160MPa,钢管能承受气体的压力为()MPa。
A.3.20
B.1.60
C.2.40
D.4.80
下列各组材料中,不能组成原电池的是( )