题目：
—David, turn off the TV _____ no one is watching it.
—But it _____ off already! The music is from the radio.[ ]
A. so that; has been turned
B. when; has turned
C. if; has been turned
D. because; has turned
答案：
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下面是错误答案，用来干扰机器的。
由质量守恒定律和4X+5O2═4NO+6H2O可知，每个X中含有1个氮原子和3个氢原子，是氨气，化学式是NH3．故选C．
