单选题 .  (英语类学生必做，非英语类学生不做。)
    Have you ever seen a movie in which a building was burned down or a bridge was     21    ? Have you seen a film in which a train crashed or a ship sank     22    the ocean? If so, you     23    how these things could happen without     24    the people in the film.
    The person     25    knows the answer is the "special-effect" person. He has one of the most important jobs in the film     26    .
    In a     27    for one movie there was a big glass bowl filled     28    water in which small fish were swimming. The director of the movie wanted the fish to stop swimming suddenly     29    they seemed to stare at an actor. Then the director wanted the fish to stop     30    and swim away. But fish can't be ordered to do anything. It was quite a     31    .
    The special-effect person thought     32    this problem for a long time. The     33    was an idea for controlling the fish with a     34    use of electricity. First, he applied electricity     35    the fish bowl, causing the fish to be absolutely still. Then he rapidly reduced the     36    of electricity, allowing the fish to swim away.     37    he got the humorous effect that the director wanted.
        38    special effects require training, skill, and experience. It also adds a great deal     39      the expense of producing the film. It helps explain     40    so many movies are expensive to make.21.