完形填空（共10小题，每小题2分，满分20分）
阅读下面短文，掌握其大意，然后从21~30各题所给的四个选项（A、B、C和D）中，选出最佳选项，并在答题卡上将该项涂黑。
When television first began to expand, very few of the people who had become famous as   21  announcers were able to be equally effective on television. Some of the difficulties they experienced when they were trying to   22  themselves to the new medium were technical. When   23   on radio, for example, they had become   24   to seeing on behalf of the listener. This art of seeing for others   25   that be announcer has to be very good at talking. In the case of television, however the announcer sees   26   with the viewer. His duty,  27  , is completely different. He is there to make sure that the viewer does not   28   any point of interest, to help him focus on particular things, and to help him   29   the images on the television screen.  30   his radio colleague, he must know the value of silence and how to use it at those moments when the pictures speak for himself.
21． A．television       B．advertisement   C．radio         D．newspaper
22． A．adopt         B．adjust          C．change         D．shape
23． A．working     B．listening      C．appearing     D．showing
24． A．practiced     B．experienced      C．determined       D．used
25． A．guarantees      B．means          C．convinces     D．warns
26． A．something      B．Everything   C．nothing        D．anything
27． A．Moreover   B．therefore      C．furthermore     D．nevertheless
28． A．miss          B．ignore         C．drop         D．catch
29． A．reveal        B．expose         C．understand       D．translate
30． A．Like       B．    Beside             C．Unlike         D．As