脊髓()
A.上端在平枕骨大孔处与脑相连
B.下端在成人平齐第2腰椎的下缘
C.呈前后略扁的圆柱状
D.有2处膨大
E.相连有26对脊神经
参考答案:A, C, D
Students shouldn't be given so difficult a problem _____ they can not work out.[ ]
A. that
B. which
C. while
D. as
简述水泵运行时轴向力的产生及危害?