机件上的肋板当剖切平面通过肋板厚度的对称平面或轮辐的轴线时，这些结构（）剖面符号。
The treatment will continue until the patient reaches the point ________ he can walk correctly and safely.
A. when
B. where
C. which
D. whose
There are several research centers in China ________ a certain disease called Bird Flu is being studied.
A. which
B. where
C. when
D. what
计算机系统软件中最核心的是______。
A. 程序语言处理系统
B. 操作系统
C. 数据库管理系统
D. 诊断程序
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