弯曲时，为阻止板料的偏移，可在坯料上增设（）。
A．加强筋
B．工艺定位孔
C．工艺圆弧
参考答案：B
When he looked up,he suddenly found himself________by a group of teenagers,________looked at
him anxiously. [ ]
A. to be surrounded; which
B. surrounded; who
C. be surrounded; who
D. having been surrounded; which
负柱面透镜可视为（）。
