单项选择题

In this section there are six reading passages followed by a total of 20 multiple-choice questions. Read the passages and then mark your answers on your coloured answer sheet.
TEXT A
The development of toothpaste began as long ago as 3000/5000 BC in the ancient countries of China and India. According to Chinese history, Huang-Ti claimed different types of pain felt in the mouth could be cured by sticking gold and silver needles into different parts of the jaw and gum. h was theories such as these that led to the development of dental cream. During the years 3000/5000 BC, Egyptians made toothpaste using a recipe of powdered ashes of hooves of oxen, myrrh, powdered and burned eggshells and pumice. It is assumed that the ancient Egyptians used their fingers to rub the mixture onto teeth.
The tooth stick, the forerunner of the toothbrush, had not, as far as is known, been discovered at this time. From the records of the ancient countries of India, China and Egypt, it was the Greeks and Romans who developed and improved toothpaste and developed a leaden instrument for the extraction of teeth. They were also the first to bind loose teeth together and to support artificial teeth by means of gold wire. During 1000 AD, the Persians gave advice on the dangers of using hard toothpowders and recommendations were made to make toothpowder from burnt hartshorn, the burnt shells of snails and oysters and burned gypsum. Other Persian recipes included dried animal parts, herbs, honey and minerals. One formula for strengthening teeth included green lead, verdigris, incense, honey and powdered flint stone.
Toothpowder or dentifrice was first available in Britain in the late 18th century. It came in a ceramic pot and was available either as a powder or paste. The rich applied it with brushes and the poor with their fingers. The powders were developed by doctors, dentists and chemists and often contained ingredients that were highly abrasive and harmful to the teeth, such as brick dust, china, earthenware or cuttlefish, and to make them more palatable, they contained glycerine. By the early nineteenth century, the ingredient stronchium was introduced, to strengthen teeth and reduce sensitivity, but it only really concentrated on the gums. In the late 18th century, borax powder was used to get the foaming effect. In 1873, an aromatic toothpaste in a jar was introduced in the U.S. and in 1896, Dental Cream was first packaged in collapsible tubes. Before the Second World War, the majority of toothpaste on the market used soap as an emulsifying agent, even though it was known that soap had certain inherent defects.
According to the passage, stronchium was most useful for______.