如1h内吸入的空气中含体积分数为（）的CO，人就会死亡。
A．1000×10-6
B．500×10-6
C．100×10-6
参考答案：A
In the past I often wrote _____ my parents about my study at school, but now I e-mail them ____ rita@sunshinetown.com.cn or call them ______ 5551 6789 .
A．for; at; on
B．to; at; on
C．to; at; for
D．for; on; to
HK系列刀开关可以水平安装，也可以垂直安装。
