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