氢气的浓度低于（）%，便不能点燃。
A、7
B、4
C、5
D、6
参考答案：B
— What's that?
— _____________ a jeep. [ ]
A. It
B. Its
C. It's
D. its
一节干电池的电压是
A．1.5伏
B．2伏
C．24伏
D．220伏
