The course of human history has seen several changes in the forms of energy. Human inventions have dramatically increased the average amount of energy     1     for use per person. The first step toward the developing of more efficient fuels was taken when people discovered that they could use vegetable oils and animal fats instead of     2     or cut wood. Charcoal gave off a more intensive heat than wood and was more easily obtainable than organic fats. The Greeks first began to use coal for metal smelting in the 4th century, but it didn’t come into extensive use until the Industrial Revolution.

In the 1700s, more energy used in the United States and other nations     3     industrialization came from renewable sources such as wood, water streams and wind etc. These were     4     obtainable supplies. By mid-1800, 91 percent of all commercial energy consumed in the United States and European countries was obtained from wood. However, at the beginning of the 20^th century, coal became a major energy source and replaced wood. Although in most regions and climate zones wood was more     5     accessible than coal, the latter represents a more concentrated source of energy. In 1910, natural gas and oil firmly replaced coal as the main source of fuel because they burned more cleanly and     6     less. Unlike coal, oil could be refined to manufacture liquid fuels for vehicles, a very important     7     in the early 1900s, when the automobile arrived on the scene.

Nowadays, fuels such as oil and natural gas provide over 82 percent of commercial and industrial energy to     8     the world economic growth. Other forms of energy derived from nuclear fission and solar power     9     for 18 percent. However, with the petroleum prices rising and the limitations in fossil fuels, energy     10     will eventually become more attractive to reduce the dependency on oil and natural gas.

For centuries, two of the most intriguing question about Leonardo da Vinci’s “Mona Lisa” were “Who?” and “When?” A discovery made at Heidelberg University in 2005 pretty much answered both. A note written in a manuscript in the library confirmed the     1     of da Vinci’s first biographer, Giorgio Vasari: that the sitter was a merchant’s wife, Lisa Gherardini. The note also helped     2     the masterpiece to between 1503 and 1506.

A third mystery—“Where?”—is still in dispute, But on June 3rd a French engineer, Pascal Cotte, declared that he and a collaborator had identified the landscape in the background of the painting. Arguments had once been made for     3     of countryside in the Marche region and between Milan and Genoa. During a presentation in Vinci, near Florence, Mr. Cotte argued that the artist was more     4     depicting a part of his native Tuscany-one that much interested him at the time. According to this theory, da Vinci     5     the area not as it was, but as, in an unrealized scheme, he intended it to be.

Mr. Cotte, who was asked by the Louvre (where the “Mona Lisa” hangs) to create a digital image of the painting, is the inventor of the multispectral (多光谱的) camera: a device that can detect not only the drawing below the surface of an oil painting, but also, where they exist, intermediate layers of work. It was among these, under what appears to be a     6     rock, that he found a preparatory sketch showing that da Vinci intended it to represent a castellated(城堡形的) tower.

The landscape of the “Mona Lisa” also includes a huge steep cliff. That is similar to one that da Vinci included in a sketch of a fortress(堡垒)     7     by Pisa and Florence in the war that broke out between them in 1503 (around the time he was painting Gherardini). The fortress with the nearby cliff—and a tower, known as the Caprona tower—all overlook the river Arno as it snakes from Florence to Pisa. All three also     8     in drawings made by da Vinci to illustrate a plan about which, says Mr. Cotte, he became “obsessive”.

Mr. Cotte argues that a channel that     9     through desolate countryside at the right of the “Mona Lisa” is too wide to be a road, as some have speculated, and is instead the dried-up bed of the Armo as da Vinci pictured it once his plan had been adopted.

It never was. But if Mr. Cotte’s theory is right, it might just explain why Gherardini, a Florentine, wears such a     10    , if mysterious, smile.