判断题
A.ActionXB.ObserverC.ObservbleD.Subscriber
A.subscribe()B.subscribe(observer)C.subscriber()D.subscribe(subscriber)
A.just(T...)B.subscribeC.from(T[])D.create
A.from(T...)B.from(T[])C.from(Iterable< ?extends T>)D.from(OnSubscribe)