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 from 53 to 62.

History books recorded that the first film with sound was The Jazz Singer in 1927. But sound films, or talkies, did not suddenly appear after years of silent screenings. From the earliest public performances in 1896, films were accompanied by music and sound effects. These were produced by a single pianist, a small band, or a full-scale orchestra; large movie theatres could buy sound-effect machines. Research into sound that was reproduced at exactly at the same time as the pictures – called “synchronized sound” – began soon after the very first films were shown. With synchronized sound, characters on the movie screen could sing and speak. As early as 1896, the newly invented gramophone, which played a large disc carrying music and dialogue, was used as a sound system. The biggest disadvantage was that the sound and pictures could become unsynchronized if, for example, the gramophone needle jumped or if the speed of the projector changed. This system was only effective for a single song or dialogue sequence.

In the “sound-on-film” system, sound was recorded as a series of marks on celluloid which could be read by an optical sensor. These signals would be placed on the film alongside the image, guaranteeing synchronization. Short feature films were produced in this way as early as 1922. This system eventually brought us “talking pictures”.

According to the passage, the phrase “these signals” refers to _________.

A. sounds             

B. series               

C. marks              

D. sensors