题目

What is it?It's called TVFrog.It is a state-of-the-art Home Theater box that can be easilyconnected to any TV set.It comes with everything already installed （安装）and is up and running within minutes.No technical know-how needed. It is an all-in-one entertainment center that allows you to get rid of all your other hardware and remotes.All the apps we know are already pre-installed like AppleTV,Amazon Firestick etc.But there is a main difference—it allows you to watch anything for free.With no subscriptions（订阅）and completely legal. How does it work?Just connect TVFrog wit如图，AB为⊙O直径，CE切⊙O于点C，CD⊥AB，D为垂足，AB=12cm，∠B=30°，则∠ECB= 度；CD= cm．