水箱浮球每隔()检查一次。
参考答案:一星期
Frank is working late again. This is the first time this week he's had to study late,_____? [ ]
A. isn't he
B. hasn't it
C. hasn't he
D. isn't it
已知1g氢气完全燃烧生成水蒸气时放出热量121 kJ,且氧气中1molO=O键完全断裂时吸收热量496 kJ,水蒸气中1 molH-O键形成时放出热量463 kJ,则氢气中1 molH-H键断裂时吸收热量为 [ ]
A.188kJ
B.436kJ
C.557kJ
D.920kJ