A、An automatic printer. B、A cheap process of developing film at home. C、A new kind of film. D、An "instant" camera that develops

The camera is often thought to be a modern invention. But as early as 1727, a German physicist discovered that light darkens silver salt, a chemical compound. Using as a camera, a big box with small hole to let the tight in, he made temporary images on the salt. Silver salt is still the base of film today.
    Then a French scientist made the first permanent picture by using a special piece of metal treated with silver salt. A photograph he made in 1826 still exists. A painter improved on the process by placing common salt the kind we eat on the meal. This was in 1839, the official date of the beginning of photography, but the problem was the printing of the photographs, and it wasn’t until other scientists developed the kind of paper we now use. The good printing was possible and photography becomes truly modern.
    In the 20th century, a man from the United States simplified film developing, and another American invented the so-called "instant" camera with serf-developing film. If we say that photography came into existence in 1839, it follows that it has taken more than 100 years for the camera to reach its present condition of technical refinement.