In order to prevent certain infectious diseases, doctors and experts often advise people: “Wash your hands often with soap and water for at least 20 seconds.” It’s common-sense advice. The surfactants （表面活性剂）in soap lift germs from the skin, and water then washes them away. Soap is a consumer product found in every household across the world. Yet few people know the long and dirty history of making soap, the product we all rely on to clean our skin.
Ancient Mesopotamians were first to produce a kind of soap by cooking fatty acids（酸性物质）like the fat from a dead cow, sheep or goat—together with water and an alkaline （碱性的）substance from wood ashes. The result was a kind of sticky substance that lifted away dirt.
Ancient people used these early soaps to clean wool or cotton fibers before weaving them into cloth, rather than for keeping humans clean. Not even the Greeks and Romans, who pioneered running water and public baths, used soap to clean their bodies. Instead, men and women kept themselves in water baths and then applied scented olive oil on their bodies. They used a metal or wood scraper to remove any remaining oil or grime.
By the Middle Ages, new vegetable-oil-based soaps, which were welcomed for their mildness and purity and smelled good, had come into use as luxury items among Europe’s most privileged classes. The first of these, a green, olive-oil-based bar soap mixed with fragrant vegetable oil, was produced in Syria and brought to Europe by Christian crusaders （十字军）and traders. French, Italian, Spanish and eventually English versions soon followed. Of these, Castile soap was the best known. The white, olive-oil-based bar soap was a wildly popular toilet item among European royals.
The settlement of the American colonies took place in an age when most Europeans turned away from regular bathing out of fear that water actually spread disease. At that time, soap was only used for household cleaning by colonists. In the new nation, the founding of soap factories like Colgate, founded in 1807, or P&Q founded in 1837, increased soap production but did little to alter its ingredients or use. Middle-class Americans had resumed water bathing, but still avoided soap. It was during the Civil War that soap was first used in bathing for personal cleaning and the practice caught on.
Today's commercially manufactured soaps are highly specialized, lab-engineered products, pleasing to the senses, but they cannot fully mask its mostly dirty ingredients, especially its petrol-based contents.
What does the underlined phrase “caught on" in Paragraph 5 mean?
A、Grew outdated.
B、Got valuable.
C、Remained useful.
D、Became popular.
【正确答案】：D
