镜头的调整，1/3inch的镜头由于其有效成像面积小，不能用于1/2inch的摄像机，而1/2inch的镜头则可用于1/3inch的摄像机。
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She _________English since she was a child.
A. learned
B. learning
C. learns
D. has been learning
I saw that she________at her homework at eleven last night.
A. was working
B. worked
C. works
D. has worked
He________trying to save a child in the earthquake.
A. was killed
B. is killed
C. has killed
D. killed
Visitors_________not to touch the exhibits.
A. requests
B. are requested
C. request
D. requested
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