A、How fax machines got a license. B、Who invented fax machines. C、How fax machines developed and worked. D、Where to buy the first

Most people might think that the fax machine, found in many offices and homes, is a relatively new piece of technology. In fact, a license for the fax machine was issued in 1843, 33 years before the telephone! A Scottish clockmaker, Alexander Bain, invented this device in the same year that the first telegraph message was being sent across America.
    Although Bain worked out the method, he never actually sent a fax transmission. It was not until 1865 that the first working fax machines and transmission service were setup. (34) An Italian man named Giovanni Casel-li developed the "pan-telegraph" which was used to send pictures and writing between French cities.
    The way that a fax machine works is quite straightforward. A machine scans horizontally across the sheet one line at a time—like your eyes when you’re reading a book. If the machine sees a white patch it records a 0, and if it sees a dark patch it records a 1. In this way the whole page is "digitized" into a string of 0s and Is. (35) A modem turns this into a series of high or low pitched sounds that are sent down the phone line to a remote receiver. At the other end, the sounds are turned back into 0s and 1s. These are printed onto a piece of paper as light or dark dots, and so the original picture is reproduced hundreds of miles away.
33. What is the main topic of the passage?
34. What was the "pan-telegraph" first used for?
35. What do we learn about the modem?