问题
选择题
I must go now, because my friend Li Lei for me. [ ]
A. wait
B. waits
C. will wait
D. is waiting
答案
答案：D
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I must go now, because my friend Li Lei for me. [ ]
A. wait
B. waits
C. will wait
D. is waiting
答案：D
.变压器的原线圈接的电压u=100
sin50πt（V），电压表为理想电表接在副线圈两端。则