数字数据网中，一般数字信道的正常误码率在（）以下，而模拟信道较难达到。
A.10-2
B.10-4
C.10-6
D.10-8
参考答案：C
The TV play _____ for ten minutes when I turned on the TV. [ ]
A. had been on
B. had started
C. had begun
D. has been on
世界上最早的国家森林公园是哪个公园？
