Why was Bazalgette's invention impressive?

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Try to imagine what life would be like if there were no electric lights, no television, or no cars. What if you had no running water to take a shower and no refrigerator to keep your food cold? Life would be tough without these great inventions.

Since ancient times, people have been inventing new machines, materials, and ways of doing things. The people who come up with new things are called inventors. Sometimes an inventor makes an invention to solve a problem. Sometimes an inventor discovers something new by accident.

Everyone probably has his or her favorite inventions. One great invention was the printing press. A German named Johannes Gutenberg in the 1400s invented a press that made it easy to print books. Books became much more common after the invention of the printing press. The books helped other inventors make new things.

The invention of the steam engine around 1700 led to the Industrial Revolution. Inventors made many new machines that were powered by steam engines. They invented locomotives, steamboats, and all kinds of factory machines.

Another great invention, the internal combustion engine, led to the invention of automobiles in the late 1800s. Inventors made better and better internal combustion engines. These are the engines we use today in cars, trucks, buses, and airplanes.

The discovery of electricity led to many wonderful inventions. Many scientists studied electricity. An American inventor named Thomas A. Edison invented the electric lamp, phonograph, and other things that ran on electricity in the late 1800s.

New things are invented today faster than ever before. Inventors are making new kitchen appliances, toys, computers, video games, telephones, and all kinds of other things.

The passage mainly ____.

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There are certain inventions and creations that people are aware of every day. There are, however, very few creations that we use our whole lives without ever thinking about or even seeing. But that is exactly what the London sewers are. Their creator, Joseph Bazalgette, may be remembered as more of an engineer than an inventor, but what he developed in London - the largest and most forward-thinking sewage system the world had ever seen - changed life in the city completely.

There was a sewage system in place beforehand, but all it did was transfer waste straight into the Thames. It was basically one massive open sewer pipe. By all accounts it created a truly awful smell, reaching its peak during the Great Stink of 1858. Even more worryingly, it caused serious health problems, such as cholera, because it contaminated the city's drinking supply. Something had to be done, and Bazalgette was the man to do it.

His solution was phenomenal: a new waste system that would divert the sewage eastwards away from the river and pump it out to sea. By 1865 most of the system was working, the main intercepting sewers used 318 million bricks and measured over 80 miles in length. To do all that work above ground would have been one thing, but to create it underground was something else altogether. The intricate maze of sewer pipes more than 100 years old was just unbelievable. And the most amazing thing is that it still functions today.

The reason is that when Bazalgette designed the tunnels, he estimated how much the population of London would increase in the next 100 years and worked out how large the tunnels would need to be to meet the needs of the future and keep the system flowing, which is why the sewers are still in working order today, and why in our eyes it should be seen as a huge inventing achievement.

Which of the following is the main idea of the passage?