燃气热水器在第一次使用是初次点火要重复数次，将管道中（）排除方可点着火。
参考答案：空气
—We don’t know who stole the book. What’s your opinion?
—Jack is considered ______ it.
A．having done
B．to have done
C．doing
D．to do
802.11a在U-NII频段上定义了（）个信道。
A.11
B.12
C.13
D.14
