单吸式离心泵由于叶轮缺乏（），工作时叶轮两侧作用的力大小不相等。
参考答案：对称性
哪种乳岩恶性度高，进展快，预后差：（）
A.硬癌
B.湿疮样癌
C.炎性癌
D.粘液性癌
E.髓样癌
There was a lot of fun at last night's party. You ____ have come, but why didn't you? [ ]
A. must
B. might
C. need
D. should
