Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.

THE ILLUSION OF FILM

Film is an illusion because the  moving pictures seen on the screen are not moving at all. A film is actually a series of tiny still pictures, or frames. They appear to be moving because the retina of the  human eye retains the impression of an object for a split second  after  that object has actually disappeared. This principle is known as the persistence of vision. When we look at a single frame of film, the image persists in the brain’s visual center for a fraction of a second. Then, the next frame comes along and the brain has to catch up with the new image. Thus, our eyes and  brain trick us into thinking that we see a smoothly moving image rather than a series of still ones.

Another  illusion of film is known collectively as special effects, the tricks and techniques that filmmakers use when makeup, costume, and stunts are still not enough to make a scene convincing. Special-effects artists apply science to filmmaking, showing us things that no plain camera could ever photograph. Even since the introduction of computer graphics in recent decades, the films of today still rely on some special effects that have existed since the early years of cinema.

The word “which” in paragraph 5 refers to ________.