Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.

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?

The word "intricate" in the passage mostly means ____.