(AFA - 2007)Read the article below carefully and a

(AFA - 2007)

Read the article below carefully and answer questions 12 to 14.

 

Fireworks

Fireworks constitute a variety of devices used for religious or entertainment purposes. They contain incendiary compounds that produce sound, color, smoke, or movement, or combinations of these. The ingredients consist of fuels, oxidizers, and modifying agents. The fuels used are charcoal, sulfur, antimony, sulfide, and powdered metals. Chlorates and nitrates usually provide oxygen for the reaction, since fireworks do not use atmospheric oxygen. Various colors are abtained from metals salts. Sodium produces yellow; copper, blue green; calcium, red; strontium, scarlet; and barium, green. Picric acid or sulfur tends to intensify the colors, and ammonium salts increase the shades obtainable. Addition of iron and aluminum powder provides sparks and fiery displays. The craft of blending mixtures packaging them is an ancient one in China, where firewoks for entertainment have been manufactured since the 13th century. In many places throughout the United States and Canada the sale of fireworks is restricted by law because of the danger of injury.

(The International Encyclopedia)

 

Change the sentence extracted from the text into passive voice.

"Chlorates and nitrates usually provide oxygen for the reaction."