Under the city streets

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.
Under the city streets
While skyscraper offices and elegant apartment blocks remain the public face of most major cities, these cities also have a mass of secret tunnels and .......(27)...... pipes below ground which keep everything working. This other world exists, forgotten or neglected by all but a tiny number of engineers and historians.
For example, there are more than 150 kilometers of rivers under the streets of London. Most have been covered over and, sadly, all that .......(28)...... is their names. Perhaps the greatest loss to the city is the River Fleet, a once great river which previously had beautiful houses on its banks. It now goes underground in the north of the city and flows into the River Thames by Blackfriars Bridge.
The London Underground has 1,000 kilometers of underground railway tracks winding under the capital and more than 100 stations .......(29)...... street level. Along some underground railway lines, commuters can sometimes catch a brief glimpse of the platforms of more than 40 closed stations which have been left under the city. .......(30)...... some are used as film sets, most lie forgotten. Some have had their entrances on the street turned into restaurants and shops, but most entrances have been .....(31)...... down.
A. Despite
B. Unless
C. Although
D. Since