Which is the digita signal?<br/> A、<img src="https://static.zxtiku.com/wangke/imgs/topic/62b96023c7543c96eed5d981a5dcaeb7.jpg"/><br/> B、<img src="https://static.zxtiku.com/wangke/imgs/topic/1b2a6c05c9ff7fc95dc0d2c6639e72f3.jpg"/><br/> C、 <br/> D、