问题
选择题
Listen! I hear someone _____ the door. [ ]
A. open
B. to open
C. opening
D. to opening
答案
答案：C
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Listen! I hear someone _____ the door. [ ]
A. open
B. to open
C. opening
D. to opening
答案：C
，该元原子核内有_______个质子，最外电子层上有___________个电子。