为什么“下利气者，当利其小便”
参考答案：后世称之为“急开支河”
分解因式：
（1）a3-a；
（2）x2-2xy+y2-1。
Helen has two sisters and a brother.Her brother is working, but _____ her sisters are still at school,[ ]
A. neither
B. either
C. all
D. both
