问题
选择题
—What does your uncle do?
—______. [ ]
A. He is a doctor
B. He is a Chinese
C. He is tall
D. She is a doctor
答案
答案：A
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—What does your uncle do?
—______. [ ]
A. He is a doctor
B. He is a Chinese
C. He is tall
D. She is a doctor
答案：A
放射源S，它向各个方向发射
，已知
，现只考虑在图纸平面中运动的