打卡子时，管道外壁有防腐层，安装卡子前应如何处理？
参考答案：
安装前应将管线漏水部位一周的防腐层按照卡子宽度处理干净。
They discovered a large collection of old pots _______ Qin Dynasty, which are still _______ today.
A．dates from; very valuable
B．dating back to; of great value
C．dated back to; of much value
D．dating from; very value
对一路信号进行FSK调制时，若载波频率为f0，调制后的信号频率分别为f1和f2(f1＞f2)，则f0、f1、f2三者的关系是 (22) 。
A．f2-f1=f0
B．f2+f1=f0
C．f1/f2=f0
D．f0-f2=f1-f0
