Helen has two sisters and a brother. Her brother is working but her sisters are still at

问题补充：

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

答案：

A. neither

B. either

C. all

(答案→)D. both

解析：both 表示的是两者都，根据上文的意思，选D，符合题意。