that many parts of Arctic Ocean do not have accurate (精确的) maps ( most maps date back to the explorations of the 19th century).

Experts suggest that there are some other passages that are better than the Northwest Passage. One of them is the so-called Northeast Passage, a waterway along the northern coast of Russia. There are no islands to sail through and climate conditions are sometimes better than in Canada. The route is already open during two months of they year.

28. What is the purpose of the Northwest Passage ？

A. To ship goods B. To transport passengers.

C. To travel underwater. D. To protect the animals in the Arctic.

29. Which of the following can replace the underlined word "permanent" in Paragraph 2?

A. clean. B. beautiful C. long-lasting D. man-made

30. What is the feature of the Northwest Passage?

A. It has fewer ice sheets . B. It has more sunny days.

C. It is open for a longer time . D. It may cost more time to pass through.

31. How many waterways are mentioned in the text?

A. Three. B. Four. C. Five. D. Six

　　　　　　　　　　　　　　　　　　　　D

Many people think of Philadelphia as the place where the Liberty Bell, a symbol of America's independence, is located. But there is another side to the city-its murals (壁画). In neighborhoods throughout the city, you will find more than 4,000 murals painted on the sides of buildings.

The murals are of different sizes, shapes and colors. Some murals are painted directly onto the buildings. Others are painted on paper and then fitted onto the wall like a puzzle.

The Mural Arts Program began in 1984 as part of Philadelphia's Anti-Graffiti Network. Today, lots of artists are painting murals with the help of community members. The program welcomes everyone to paint and is now working with more than 25,000 local people of all ages and backgrounds. A mural called "Finding Home", for example, was created by homeless people.

Artist Meg Saligman, who has painted many murals around the city, says that she is honored to take part in the program and that she hopes it can bring changes.

"Spending a couple of hours painting with others is a great experience," says Deborah O'Brien, a Philadelphia volunteer. The murals, she adds, represent the diversity and sense of community.

Murals can last anywhere from 5 to 25 years. When a mural starts to fade(褪色), the program tries to save it. "People are sad and angry when you take a mural away because people really love them," says Saligman.

Mural subjects range from sports to politics to history. That means everyone can have a favorite. If you're planning to visit Philadelphia, be sure to check out the colorful murals.