自备发电机组的中性线应单独接地，其接地电阻不得大于（）欧。
A、4
B、10
C、20
D、30
参考答案：A
—______ two little boys behind the trees?
—No, ______. [ ]
A. Is there; there is
B. There isn't; there are
C. Are there; there aren't
D. There aren't; there aren't
