A、A platform worker. B、A computer. C、The command spot. D、A machine. D选项均为表人或物的名词词组表明，本题可能考查动作的施予者。短文中提到Here a machinechecks and

The world’s first completely automatic railway has been built under the busy streets of London. The railway is called the Victoria Line, and it is part of the complete London underground railway.
    The new Victoria Line was opened in 1969. This new line was very different from the others. The stations on the other lines need a lot of workers to sell tickets, and to check and to collect them when people leave the trains. This is all different on the Victoria Line. Here a machine checks and collects the tickets, and there are no workers on the platforms.
    On the train, there is only one worker. If necessary, this man can drive the train. But usually he just starts it: it runs and stops by itself. The trains are controlled by electrical signals which are sent by the so-called "command spots". The command spots are the same distance apart. Each sends a certain signal. The train always moves at the speed that the command spots allow. If the command spot sends no signals, the train will stop.
    Most of the control work is done by computers. The computers also fix the train’s speeds, and send the signals to the command spots.
    Other machines make sure that the trains are always a safe distance apart. One train may stay too long at a station: the other trains will then automatically move slower. So there is no danger of accidents on the line.
Questions 19 to 21 are based on the passage you have just heard.
19. What happened to the complete London underground railway in 1969?
20. Who does all the checking and collecting of tickets on the Victoria Line?
21. What is the task of the only worker on the Victoria Line train?