单项选择题

A television camera does not look at a scene as a whole in the same way as a film camera; instead, it scans the scene. Scanning was first used in the transmission of pictures by telegraph. A light beam looked at a very small part of the picture and translated what it saw into an electric current, which varied in strength according to the amount of light which passed through the picture at that point. It then moved on to the next tiny part of the picture, working across it from left to right, then moving down a line and repeating the process, until it had scanned the whole picture. At the receiving end, the signals were re-translated into a series of dots which, when looked at from a normal reading distance, reproduced the original picture. The television camera also looks at a scene as a series of dots; each dot of light is translated by the camera tube into an electronic signal.
The black and white television receiver—the set in your living-room—must re-translate the information received from the camera into a series of dots of varying brightness to make up the picture. An electron gun shoots a beam of electrons at a screen coated with a substance which lights up when the electrons hit it. Only one "dot" of the screen lights up at a time as the electron beam moves across from left to right in a series of lines, but the movement is so quick that the viewer watching the other side of the screen sees only the complete picture. The number of lines in the picture varies according to the system used. In Britain, the B.B.C. used 405 lines for many years, but is now also using 625 lines. Soon, all television in Britain will be on 625 lines. The picture of 625 lines, known as a "field", is transmitted in Britain at the rate of fifty fields per second.
Which of the following statements is true