问题
选择题
There _____ a pen and some books on the desk. [ ]
A. have
B. has
C. is
D. are
答案
答案：C
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There _____ a pen and some books on the desk. [ ]
A. have
B. has
C. is
D. are
答案：C
应带正电
应带正电
应带正电
应带正电小题2:如果只在偏转电极XX/上加如图①所示的电压,试在图③中画出所观察到的波形图；