Until men invented ways of staying underwater for more than a few minutes, the wonders of the world below the surface of the sea were almost unknown. The main problem, of course, lies in air. How could air be provided to swimmers below the surface’ of the sea Pictures made about 2,900 years ago in Asia show men swimming under the surface with air bags tied to their bodies. A pipe from the bag carried air into the swimmer’s mouth. But little progress was achieved in the invention of diving devices until about 1490, when the famous Italian painter, Leonardo da Vinci, designed a complete diving suit.In 1680, an Italian professor invented a large air bag with a glass window to be worn over the diver’s head. To "clean" the air a breathing pipe went from. the air bag, through another bag to remove moisture, and then again to the large air bag. The plan did not work, but it gave later inventors the idea of moving air around in diving devices.In 1819, a German, Augustus Siebe, developed a way of forcing air into the head-cover-ing by a machine operated above the water. At last in 1837, he invented the "hard-hat suit" which was to be used for nearly a century. It had a metal covering for the head an air pipe attached to a machine above the water. It also had small openings to remove unwanted air. But there were two dangers to the diver inside the "hard-hat suit". One was the sudden rise to the surface, caused by a too great supply of air. The other was the crushing of the body, caused by a sudden diving into deep water. The sudden rise to the surface could kill the diver; a sudden dive could force his body up into the helmet, which could also result in death.Gradually the "hard-hat suit" was improved so that the diver could be given a constant supply of air. The diver could then move around under the ocean without worrying about the air supply.During the 1940s diving underwater without a special suit became popular. Instead, divers used a breathing device and a small covering made of rubber and glass over parts of the face. To improve the swimmer’s speed another new invention was used: a piece of rubber beathing pipes made it possible for divers to float on the water, observing the marine life underneath them. A special rubber suit enabled them to be in cold water for long periods, collecting specimens of animal and vegetable life that had never been obtained in the past.The most important advance, however, was the invention of a self-contained underwater breathing apparatus, which is called a "scuba".Invented by two Frenchmen, Jacques Cousteau and Emile Gagngn, the scuba consists of a mouthpiece joined to one or two tanks of compressed air which are attached to the diver’s back. The scuba makes it possible for a diver-scientist to work 200 feet underwater or even deeper for several hours. As a result, scientists can now move around freely at great depths, learning a- bout the wonders of the sea. The German, Augusts Siebe, invented the "hard-hat suit" which was used().

A. for nearly a hundred years
B. for almost a thousand years
C. for over a century
D. for a century