水动力不稳定的原因是（）和（）两者之间的密度不同。
参考答案：蒸汽；水
One of the tennis rackets broken when they found them.[ ]
A. is
B. are
C. were
D. was
包装容器钢提桶的许可证发证检测，堆码试验的堆码载荷为（）N。
A.860
B.1250
C.1380
D.1520
