Section B
Directions: In this section, you will hear 3 short passages. At the end of each passage, you will hear some questions. Both the passage and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A, B, C and D.
听力原文： Today, I'll be talking about the invention of the camera and photography. The invention of the camera and photography changed our lives a lot. The camera is often thought to be a modern invention. But as early as 1727. (26) 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 light in, (26)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 sensitized with silver salt. A photograph he made in 1826 still exists. The painter, Daguerre improved on the process by placing common salt on the metal. (27)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, that good printing was possible and photograph became truly modem.
(28) In 1860s, Mattew Bradey was able to take his famous pictures of the American Civil War. In the 20th century. George Eastman of the United States simplified film developing, and Dr. Edwin Land invented the so-called "instant" camera with self-developing film. If we say that photography came into existence in 1839, it follows that it has taken more than one hundred years for the camera to reach its present condition of technical refinement.
(27)

A. Light darkens silver salt.
B. Light darkens natural salt.
C. Light darkens silver.
D. Light darkens self-developing film.