南京地铁DC1500V直流开关是（）断路器。
A、电保持型
B、磁保持型
参考答案：A
There ______ snow in Canada every year. [ ]
A. has many
B. has much
C. are many
D. is much
电视矩阵所切换的信号是指（）.
A.视频
B.音频
C.中频信号
D.射频信号
