How does the top-down approach differ from the bottom-up approach in language processing (对外经贸2005研)

A．Bottom-up processing is more effective than top-down processing in speech synthesis.
B．Bottom-up processing utilizes all information one has in speech perception.
C．Top-down processing uses one’s existing knowledge and the incoming speech signal.
D．Top-down processing is more efficient than bottom-up processing in speech comprehension.