Read the following passages carefully and answer the questions that follow. There was great excitement on the planer of Venus this week. For the first time Venusian scientists managed to land an unmanned spacecraft on the planet Earth and it has been sending back signals, as well as photographs ever since. The craft was directed into an area known as Gonebay named after the great Venusian astronomer Professor Gonebay, who first discovered it with his telescope 200000 light years ago. Because of the excellent weather conditions and extremely strong signals, Venusian scientists were able to get valuable information on the feasibility of a manned spaceship's landing on Earth. A press conference was held at the Venus Institute of Technology. "We have come to the conclusion based on last week's landing, Professor Bright said, "that there is no life on Earth," "How do you know this?” the Science reporter of Venus Evening Standard asked. "For one thing, Earth's surface in the area of Gonebay is composed of deadly soil and nothing new can grow there. For another, the atmosphere is filled with carbon monoxide and other deadly gases and nobody could possibly breathe this air and survive." "But, Professor Bright, the photographs indicate some movement and possibly there are millions of crawling beings." "They may be crawling but they are not living, according to the Venusian Standards. These crawling objects, called "human beings' inhale and exhale such deadly gases that they lost the power to live long, long ago." "What will happen if the Venusians land there?" "Certainly they will die. We shall have to take our own oxygen with us, if we are to land our people in any of the concrete jungles on Earth." In the sentence "There are concrete jungles on Earth", the phrase 'concrete jungles' means