问题
选择题
Boys and girls, try not ______ in class.[ ]
A. sleep
B. to sleep
C. sleeping
D. sleepping
答案
B
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Boys and girls, try not ______ in class.[ ]
A. sleep
B. to sleep
C. sleeping
D. sleepping
B
，正电荷
，正电荷
，负电荷
，负电荷