对于气焊焊口，通常采用的热处理方法为（）。
A．正火处
B．高温回火
C．正火加高温回火
D．去应力退火
参考答案：C
---How long have you been studying at this school?
--- I have been studying here____.
A．since I was thirteen years old
B．when I was thirteen
C．at the age of thirteen
D．after I was thirteen
数据通信系统抗干扰性明显（）于模拟通信系统，而数字通信比模拟通信在信道上所占用的频带要（）
