Watercolour is the oldest paints known. It dates back to the early cave men who discovered they could add lifelike qualities to drawings of animals and other figures on the walls of caves by mixing the natural colours found in the earth with water. Fresco  , one of the greatest of all art forms, is done with watercolour. It is created by mixing paints and water and applying these to wet plaster  . Of the thousands of people who stand under Michelangelo's heroic ceiling in the Sistine Chapel, very few know that they are looking at perhaps the greatest watercolour painting in the world. The invention of oil painting by the Flemish masters in the fifteenth century made fresco painting go down-hill, and for the next several centuries watercolour was used mainly for doing sketches   or as a tool for study. It was not until the eighteenth and nineteenth centuries that English painters put back watercolour as a serious art form. The English have a widely-known love for outdoors and also small private pictures. The softness of watercolour had a remarkably strong attraction for them. The popularity of watercolour continued to grow until the twentieth century. The United States passed England as the center for watercolour, producing such well-known watercolour artists as Thomas Eakins and Andrew Wyeth. According to the passage, watercolour painting was put back in England because   _
it was easy to use outdoors it was a strong medium it was extremely bright in colour it was suited to popular tastes