问题
选择题
— What is he doing?
— He ____ back to class. [ ]
A. runing
B. running
C. is running
答案
答案：C
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— What is he doing?
— He ____ back to class. [ ]
A. runing
B. running
C. is running
答案：C
，其中Ω是由曲面z=x2+y2，z=0及x2+y2=1所围区域．